Why did Avraham" fall upon his face"?
Ramban: To prepare to receive a prophecy (both here, and again in 17:17, where he prepared for the next prophecy). 3
Seforno: To accept the covenant that Hashem had made with him, and to thank Him for it.
Chochmah u'Musar (Vol. 2, p. 338): Even though Avraham was waiting for Hashem to command him, lack of the Mitzvah harmed him, just like natural dangers harm even b'Ones. All the more so, how much sin blunts a person!