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 HaSh-m had made with him, and to thank Him for it.
Chochmah u'Musar (Vol. 2, p. 338): Even though Avraham was waiting for HaSh-m 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!