When I wear a backpack on a bike, then yes, definitely. It helps to keep the weight at the waist rather than on the shoulders, although it's not a important as when hiking, and keeps the pack in place, prevents side-to-side movement. It's also less beneficial for short commutes than for trail riding.

I actually bought one of those new Camelbaks that puts the water bladder down, horizontally around your waist, for mountain biking. A big difference. I actually started wearing in on long weekend road rides.