Why is Jesus called the "Son of Man"?

Let me give a common understanding and then a more sophisticated historical understanding.
The common understanding is that "Son of God" implies his deity—which it does—and that "Son of Man" implies his humanity, which it does too.

He was a son of man, that is, a human being. And he is the Son of God, in that he has always existed as the Eternally Begotten One who comes forth from the Father forever. He always has, and he always will. He is the Second Person of the Trinity with all of the divine nature fully in him.

He is born of a virgin. He had a human father but he didn't have sex with this virgin until Jesus was conceived. He was conceived of the Holy Spirit in the virgin Mary. Thus he is human—fully human. The Bible wants to emphasize that he is fully human.