That said, I keep finding more AppData files in my Documents folders than the other way around. Linux software (especially legacy software that doesn't respect the XDG spec) is the worst for that, somehow spreading files and directories across my user profile directory and my documents directory.
Seemed like a bunch of disparate things with the most common probably being a dumping ground for programming language library caches (Maven and NuGet and maybe another couple).
Let's just call it what it is: a complete mess. Some apps install their binaries in there, some just config files, some all your data.
If there are guidelines (and they are not conflicting) about 1/3 of apps are ignoring them.
Also good luck trying to back it up with robocopy or anything, maybe windows' own backup tool can read it while running on that account, I usually get errors, which I think are mostly fine to ignore.
Which programs do that? Or do you think things like preferences for programs should live among your documents?