As far as I remember, the reason it was changed was because if the avatar gif was called anything you like, as "userId_mydog.gif", when you wanted to upload a different pic as "userId_me.gif" you´d end up with two avatars on the system. This means you could end up uploading hundreds of avatars, taking server system resources.
If the system then deleted "userId_mydog.gif" when uploading "userId_me.gif" then all previous links would be broken. On the other hand, now these are called dynamically, so you could probably start referencing the new gif url rather than making url static and only changing the file itself.