Why do people pray to Buddhas?

Why do Buddhists pray to Buddhas in temples? I heard that Sakyamuni didn't approve of that and even criticized Bodhisattva for telling other people that he can help them with things that only God can help with. Sakyamuni considered himself and Bodhisattva as ordinary human beings without any divine abilities. Is that true? Is there any reference or source which confirms this?