Well, you could do that for a lot of situations actually. If you want to separate your subjects into blogs (one for gaming and one for science for example). If you want to have a separate account for DTube, Utopian, DLive, DSound, etc... If you want to have a blog in english and the other one in your mother tongue. Those are the use cases I can think of on the spot, more probably exist though.

I have currently 3 accounts : this one for blogging, one for testing interactions with the Steem blockchain and one for my Steem related project (which hasn't been used yet, just wanted to reserve the name).