I don't use Facebook, but mobile apps tend to add features compared to the web app, such as notifications, integration with other apps (e.g. contacts), and a cleaner UX.
That being said, even if I used Facebook, there's no way I'd install their app because I don't trust them.
Indeed mbasic.facebook.com works, but it doesn't look as good and doesn't support message notifications. It's a trade-off I'll take for now, but not everyone will.
That being said, even if I used Facebook, there's no way I'd install their app because I don't trust them.