Why do gorillas engage in knuckle-walking when its a primate like humans?

Humans have 2 legs and 2 arms. So do gorillas. They are biologically very similar to humans and have 98% similar DNA too. But gorillas sometimes do knuckle-walking, while humans don't. Why? Their legs do work and they can stand and walk straight. For humans, knuckle-walking would be a big pain and discomfort.