Putting things I need in the bag, in addition to the large volume of water particularly appeals to me. I found out the hard way recently, in very hot weather, that water and energy were very, very important when cycling for more than an hour. I've learned to make my own energy bars now so plan to take a few with me on every ride to avoid going hypoglycaemic.

I also travel a lot and the backpack could come in useful on some trips.