Why do flies rub their limbs together?

(WHTM) --We have all seen flies do it. They land on a surface, stay perfectly still, and then rub their limbs together like a supervillain that wants to dominate the world.
Well, that last part may not be true. But, why do flies rub their limbs together?
It may sound like an oxymoron, but, they are actually cleaning themselves.Raid.com says that flies have sensors all over their body. On their feet, as well as tiny hair-like features all over their body. It may seem weird that an insect that thr