Vimscript is used to customise the behaviour of your editor, and the uses to which you can apply it are literally endless.
Note that the boundary between Vim configuration and Vim scripting is pretty fuzzy: in many respects, if you have a
.vimrc file, then you have already started learning Vimscript, because Vimscript is what that file contains.
Note also that a plugin is just some Vimscript neatly packaged up, often (but not always) so that it can be distributed and used by other people. Many plugins started out as just a bit of Vimscript in someone's .vimrc, and then gained functionality and grew until the author decided to split them out into a plugin.
So yes: if you ever want Vim to do anything differently from how it would if you started it with
vim -Nu NONE, then there is lots of benefit to learning Vimscript, even if you don't intend to develop and release any plugins yourself.
Plus it's just fun.
On the other hand, your question about whether anyone uses Vimscript for any projects that aren't related to Vim development doesn't really make much sense. There are no interpreters for Vimscript other than Vim, so anything you develop in Vimscript must run inside Vim, even if it's not strictly speaking a plugin.* This implementation of the game Snake doesn't really have anything to do with editing text, but it is still a Vim plugin.
And while Vimscript is Turing complete and thus could in theory be used for pretty much anything, in reality it's very tailored for the task of customising Vim's behaviour, and has many rough edges, so its utility for building code that is completely unrelated to Vim (other than using it as an installation platform) is pretty limited.
* If you wrote 10,000 lines of Vimscript and told anyone that wanted to use it to copy and paste it into their .vimrc, that's not a "plugin".