I have a question about building a deck.?

So i'm building a deck at my house. It's a pretty simple design, a free-standing, low-profile deck with the 2x6 joists suspended between the beams using joist-hangers, instead of resting on top. The plan I'm following calls for 1/4 inch gaps between the ends of the joists and the beams, instead of having the joists butt up against the beams. Why? I've already completed this step, and I'm all set to lay down the planking, but I just want to understand why it's supposed to be this way.