The POSIX standard is more portable. If you are writing scripts for your system, you can use the full features in the main man pages. If you are writing code that you want to run on other Linux systems, maybe with reduced feature sets like a tiny embedded computer or alternates to gnu tools like alpine linux, or even other unixes like the BSDs, you will have a better time if you limit yourself to POSIX-compatible features and options -- any POSIX-compatible Unix-like implementation should be able to run POSIX-compliant code.

This is also why many shell scripts will call #!/bin/sh instead of #!/bin/bash -- sh is more likely to be available on tinier systems than bash.

If you are just writing scripts and commands for your own purposes, or you know they will only be used on full-feature distributions, it's often simpler and more comfortable to use all of the advanced features available on your system.

If you execute a binary without specifying the path to it, it will be searched from the $PATH environment variable, which is a list of places to look for the binary. From left to right, the first found one is returned.

You can use which cat to see what it resolves to and whereis cat to get all possible results.

If you intentionally wants to use a different binary with the same name, you can either directly use its path, or prepend its path to $PATH.