I can think of a few reasons.
If your various depts instest on having thier own sepererate servers. HR and finance have thier own stuff and do not want others in them. However if your a SA, you can get into any place you want. So they get a really powerful box and put two instances. Then they can have thier own SA.
Another would be if your a service provider, you could give everyone thier own "server".
Im sure there are others, but ive had to deal with the first reason.