I'm still playing Neverwinter Nights on Arelith server, here's a great quote from the forums:
"ARELITH: For when you like D&D, but you just don't want those people in your house."
--Coldend from Arelith Forums
Brilliantly said, Coldend.
In actuality, I'd say the true advantage is ease of grouping with other players of a similar interest, a graphical and rules interface, and a way of storing up a world without a huge binder of loose pages.