I just realized I hardly ever have two windows side by side on OS X, why would I do it on an iPad? There are occasions of course, and having that option is definitely a plus.
There are so many uses. For me, I run a blog for a sports team and I can have Twitter open following social media for the game and the notes app open writing about what's happening for a post I'm making. Or I can have Wordpress open in Safari (or the wordpress app when it's supported) and the stats open in the NHL app.
It's such a great feature and I'm super pumped for 3rd party devs to support it.