2 Reasons:
1. People can see their GUID, it think most people have trouble with it, when they someday need it (unbans, adminapps etc)
and 2. Most important what was mentioned a post before, you can see if some troll is rejoining, you can see, if a famous person, that joins the server, really is this guy, or a fake.
I use it on every of my servers and I am quite happy with it