Why is weight loss important for people with osteoarthritis?

They will feel bette. For every pound you weigh, you put 4 times your body weight across each knee with each step. So if you weigh 300 pounds, each knee is subject to 1200 pounds of force with each step. Therefore, if your cartilage is not normal (arthritis) it will suffer more stress due to the force (weight) it must deal with. On the flip side, even modest weight loss can significantly improve your pain.
Wt. worsens symptoms. A person's back, hips, knees, and feet take on a lot of stress when he carries a heavy backpack or gains 50-100 pounds in weight. Obesity can worsen joint problems such as pain, swelling, difficulty walking, or trouble climbing stairs. Even if the original cause of the joint problems were from other diseases or injuries, the heavier a person is, the more wear and tear the joints have to endure.
Less wear and tear. Osteoarthritis patients that have pain in the lower back, hips, knees or ankles, could have a reduction in pain with weight loss. This is related to less 'wear and tear' on these joints as they go about their daily lives.
All about weight. OA generally hits the weight bearing joints. If those joints are bearing less weight they'll generally hurt less and the OA therein will progress less. So weight loss is helpful.