If we had no way of detecting between browsers that would lead to a much more consistent cross-browser experience in the long run and not allow one browser's dominance to affect the internet experience for the rest.
- identifying active sessions
- working around bugs in specific versions of a browser/os
- changing UI to match the native experience of a user
- seeing percentage of users using different browsers (which is important to know if you can use new features in web apis)
- etc.
First step would be for all browsers to send the same UA. Once it becomes completely meaningless, nobody will read it any more and we can drop it permanently.