What is the meaning of the Dua given in this Hadith?

Imam Ahmad and others narrated with a saheeh isnaad from ‘Uthmaan ibn Haneef that a blind man came to the Prophet (blessings and peace of Allah be upon him) and said: Pray to Allah to heal me. The Prophet (blessings and peace of Allah be upon him) said: “If you wish, I shall pray for you; and if you wish, I shall delay that for you and that will be better for you.” [According to another report, he said: “… Or if you wish, you can be patient and that will be better for you.”] He said: Pray for me (now). So he instructed him to do wudoo’ and do it well, then to pray two rak‘ahs and say this du‘aa’ (supplication): “O Allah, I ask You and I turn to You by virtue of Your Prophet Muhammad, the Prophet of mercy. O Muhammad, I turn by virtue of you to my Lord concerning this need of mine, that it might be met for me. O Allah, accept his intercession concerning me and accept my intercession concerning him.” So the man did that and he was healed. What does it mean when the man is ordered to say "O Allah(...) I turn to You by virtue of Your Prophet Muhammad" I turn to you by virtue of Prophet Muhammad?? And also "O Muhammad, I turn by virtue of you to my Lord concerning this need of mine, that it might be met for me" Why should the man call out to Muhammad (pbuh)? What do these mean and what is the reasoning for the choice in using these words?