That to me is a very necessary feature, for at least two reasons.
Internationalization: URLs are always latin chars, which would exclude any non-latin community or usernames.
More complex community names. You can see how ridiculous reddit community names can get, because they don’t allow spaces or non-latin characters, in either community or usernames. Spaces specifically is a necessary one for both, and URLs wouldn’t permit that.
That to me is a very necessary feature, for at least two reasons.