Aerodynamic effect of drafter to the draftee
When you are riding along on your own you're not only pushing the air in front of you out of the way, the most obvious source of air resistance, but you're also being slowed by the turbulent air behind you. However slightly.

Surprisingly, having someone draft you will help smooth out that turbulence, and so you benefit as well! ( Just make sure they take their turn in front ).

http://www.exploratorium.edu/cycling/aerodynamics2.html