Being able to select arbitrary text is a feature pretty much exclusive to websites and website-derived apps (ie electron apps). "Normal" programs and "native" phone apps or games generally don't let you select most text, or don't even implement the concept of text selection at all.
So in the eyes of some designers, disabling arbitrary text selection therefore makes your webapp feel more like a "proper" app and be perceived as a higher-quality thing. And indeed, disabling text selection on things like buttons or menu items indeed may make the experience slightly less janky; the buttons on this very site do this for example.
In practice, being able to select arbitrary text tends to be very useful. Windows error messages for example can simply be copied with Ctrl+C, but since the selection is invisible it's not something that's obvious to the vast majority of users.
So, before you go ahead and
user-select:none everything for better feels, make sure you keep it enabled for any elements which the user may want to take with them elsewhere - into translation engines, searches, chats with friends, and so on.