Wading belts give us overweight guys a nice, svelt look.
Seriously, wading belts help keep water from filling into the legs of waders. If you fall into a stream and water fills your waders, you need to be able to pull your own body weight plus the weight of the water to get out.
I have taken a few falls. My worst case was that a little bit of water seeped in, but not enough to weigh me down when I regained my footing.
In theory they keep your waders from filling up with water. I find they also give you lower back support, I don't know bout you guys but a long day of fishing can be murder on my lower back. I'm glad we are just about into wet wading season, I think wet wading puts one more in tune with the stream enviroment.