User-Agent for browsers is pretty much useless now, but it's still a pretty useful name for a header if you want to put real information in it. Ex, if you're calling an api, it's nice to have a version number and a name/contact so broken things can be fixed.

I don't remember if HTTP/2 (and presumably /3) has a initial dictionary for header compression, they could have a suggested value in there, so if you want to look like a browser, you can use that at a low byte cost.