Burn off more calories,
Strengthen hips and knees,
Back ache will be less likely and will also prevent back ache,
Free exercise instead of going to the gym.
How to bend properly (squat):
When you bend, keep your upper body upright. When you are bending with feet side by side (squat bend), keep both heels down and your weight back over your heels to keep your weight on your leg muscles and off your knee joints. Don't stick your behind far out in back.
So the next time you need to bend down to put something in the fridge or pick up the paperclip you've dropped do a squat instead.