I won't argue that it's mostly useless (because you can't use without looking at it), but here's quite a few usecases:
Quckly adjust brush size and a ton of other things in Affinity Photo, Publisher and Designer (I use it all of the time, way better than looking for small UI menus), text suggestions for people who can't type quickly, bold/italic/underlined/etc. text in Microsoft Word/Excel/PowerPoint/Apple apps without learning the shortcuts, shortcuts replacement in Final Cut and other Apple apps, skipping ads on YouTube and controlling media playback. Maybe there's way more things that I can't remember. Also it is way more interactive than a row of Fn keys that could... Change the level of brightness?