From my experience, if the first layer doesn’t come out well, the print will fail.

It comes from a lot of things

A giant one layer sheet is obviously a torture test. But it is also an exercise to properly understand what your print bed ACTUALLY is. Printing across your entire bed can be a bit of a mess due to printers/firmware that reserve space for printing a test line or wiping the nozzle and so forth.

Whether it is worth a full 256^2 mm of filament is a different question

Also... there is the question of how big of a part people actually need and if you aren't better off printing that in multiple segments regardless.

And... it also isn't necessarily a good test of this in the first place. Because the "auto leveling" process already compensates for stuff like this and actually getting a fully intact sheet to come off the plate and be comparable isn't easy on even a perfectly flat bed, let alone one where the contours were compensated for (to whatever degree). Which makes it VERY hard to tell if you truly had a perfectly flat sheet or if there was distortion because your lower left screw is too tight.

But the first layer obsession kind of goes back to the early days of printing when there WAS no "auto leveling". Already alluded to it, but at a VERY high level, "auto leveling" creates a virtual mesh of the printbed and adjusts the nozzle height (z-axis) in real time. For any "kind of level" printbed, that is more than good enough since the variance for a single "part" is going to be negligible unless you are doing almost the entire printbed anyway.

But, in general, it is good practice to ACTUALLY level your bed and compute a proper z-offset. Which, again, goes back to the early days and The Paper Trick. Which, like most things FDM, was a closely kept secret because it was a great way to identify the newbies and be gatekeeping pricks to them. The importance of getting super precise (while understanding little to nothing about precision...) is a lot less important, but it is still a good way to show off that you are "a real printer" by doing it yourself to the most exacting level possible and then posting to the 'gram about it.

And speaking of the 'gram, the last part is just that everyone wants to be an influencer. Someone like Angus or Michael will do it as part of a printer review because they are showing off how good an advertised feature is or demonstrating fundamental build quality issues. But people don't pay attention and just decide they are also a cool ass youtuber 3d printing expert and want to do it themselves.

I think there is definitely value in doing a full bed print IF you need to. But as a "normal" calibration test?

I think it depends on what you want to print. Personally, most of my prints fit within a much smaller footprint. So, I don't usually need my first layer to be perfect across the full bed. However, when trying to print something larger, I can absolutely tell how terrible my first layer is. It sucks to end up reprinting the first layer half a dozen times, because one small area keeps failing to adhere.

A large part of my issue is the printer I have just isn't all that good. And, when I get less lazy, I'm going to just build a Voron. At that point, I'll probably be one of those folks tweaking it until I get a perfect first layer. Because I want to be able to start a print and not spend the next hour fighting first layer problems.

This fixation isn't new at all. One of the most popular modifications to Prusa printers for years has been the Nylock mod to make your plate as flat as possible.

It does make sense, while most parts don't take up the whole bed area, when you do need to utilize the entire space of a printer you dont want to have to struggle with it. Having a good first layer maximizes the chance that your print will be successful.

It's to test the mesh bed level: https://all3dp.com/2/mesh-bed-leveling-all-you-need-to-know/

I inherited an older Delta printer that had a warped, janky bed and damaged nozzle from poor calibration. Even after replacing damaged parts and recalibration, I still need to print rafts for everything. That first layer is everything, and the tutorials for getting an even first layer have been immensely helpful in my journey.

Definitely nothing new, that's been going on since before I got into 3d printing like 7 years ago.

There was a point where having a good first layer was actually critical to having good print bed adhesion and successful prints. 99% of problems were solved by re leveling the bed and/or cleaning it.

Now, with abl and fancy tools like lidar and better bed surfaces, it's not nearly as essentially to get a perfect first layer, but it's still a sign of a well calibrated printer.

I dare say it's one of those things that's just hung around because as waves of new users join the community (especially around times like Xmas when a lot of new people join), they start researching stuff and see older users posting about their first layer and think of it as some kind of rite of passage, so they post theirs.... And then the next group come in and see it, wash rinse repeat....

Idk. I've been fine just printing a thin perimeter line to see if it's level. And maybe a smaller thing to see if I get the correct amount of squish for the first layer. I don't think there's any benefit in having it plaster the whole bed for 30 minutes.

People who do first layer tests across an entire build plate are being anal retentive. Or they are looking for some kind of bragging rights.

Thanks to the various auto bed leveling systems out there on every printer these days, it means very little to do such extreme tests. And as long as your heat bed is flat enough to be in compensation tolerance for the auto leveling system you are using, you will never notice much.

This is not to say that it can't be a diagnostic tool if you ever need to check for a warped heat bed. But that's pretty rare these days to find a heat bed that's that bad. And if you are running Octoprint, there is a plugin to map the flatness of your build plate that you can see.

*** A word of caution: One needs to understand that the "flatness" of your build plate can and will change at various temps. Just because it might appear flat at one temperature, does not mean it will hold that flatness at a different temperature.

Prusa's first layer calibration in the past would do a long line across the plate to give you time to adjust, I personally just use a piece of paper or feeler gauge (have a tap probe) to set my offset and then run with it. Auto levelling and meshing work extremely well in my experience, if you have something adjustable imo you're best doing that offline anyhow, the nozzle to surface distance is what matters, you don't need to push plastic to measure that, in fact I wouldn't even attempt to do that until I was confident in my measured offsets, tool crashes suck and super close scraped on plastic sucks to remove from a surface.

Its just a test to dial your printer in. I shimmed my bed with 0.1mm washers. I haven't done a full square of plastic, but I printed my first layer / z-offset print of choice in all 4 corners and center in order to verify the bed level results in octoprint were accurate.

Before hand 70% of my bed printed perfect, but one spot was a little lower, and the mesh bed leveling wasn't accounting enough for it. Parts printed on textured sheets would not pick up the texture as well in that one spot. I like the textured look for top surfaces of control panels and such, so having an area on the bed that wouldn't apply the texture was a bit annoying.

Tests like what you are talking about is an extreme way to verify that everything is square, or at least well accounted for in the firmware.

Also, since this wasn't something achievable out of the box until recently, printer manufacturers are showing it off as a point of pride / as a sales tactic.

Not sure I'm taking about the same thing but what I've seen is people printing a couple of layers out of pla then switching to other material to avoid having to mess with glue or else. This is to avoid bounding with plate as it can happen with petg or other material. But in no case I've seen people printing entire build area, usually the same size as print + a bit of extra to pull off that pla layer

I didn't know this was a thing, but I hope 3d printer reviewing people adopt it as a standard test. Most of the time a prints fails for me, it's because of a inconsistent (continuously changing) first layer.