They're generally useful in promoting temporary content such as festivals. In places such as Instagram they're found their use in being used for casual posts that aren't of the quality of the users more cultivated actual posts. For example promoting a friend's page or casual holiday pictures. Particularly if the account has a brand such as hand made goods, it's a place for the owner to post personal content too.

I don't actually know what PixelFed is really but I currently only use web content where I don't really follow people as much as ideas and content, I couldn't name a single lemmy or Reddit user, I only follow hashtags on Masterdon and I don't really have much loyalty to who I watch on YouTube, so this content doesn't appeal to me because I don't often care about their stories. If PixelFed is a content site where you do follow personal content, then it's probably right for them.

Its from snapchat, it was once a unique feature of them to encourage people to make more authentic, spontanious posts. Because they are deleted after a certain time, people (at least in theory) don't think too much about making them.

It's mostly for things that don't warrant a post but your friends might still think are interesting. I'm big on live music so it makes it easy for me to see a snippets of shows people go to or even to figure out if friends happen to be at the same show, but that's just my personal example.