I used to be heavily into hiking and backpacking, before I got into biking to save the wear and tear on my knees and ankles. As I recall, the rule of thumb for a backpack is if you are going to be carrying more than about 20 lbs, you want a hip belt. As with bike gear, proper fit is important for a backpack. The hip belt has to sit on your pelvic bones to distribute the load directly to your legs and not to your back. The sternum strap functions to keep the shoulder straps properly positioned and not to slide outward. There are usually a bunch of other straps to compress and support the load to keep it from flopping around while you're climbing over boulders and such.

That said, for biking, I've never worn anything bigger than a Camelbak HAWG which is pretty tiny by backpack standards. I used to use the hip belt with it, but later took it off and I can't say I've noticed much difference (probably because it's crap anyway, compared to a real hip belt like on my Granite Gear Nimbus pack). I did like the sternum strap though, until that broke on me so now I don't use that either. But I would still use the sternum strap if I could. You definitely don't want the pack flopping around while cornering or on rough terrain.