The Midrash Rabbah (39:1) relates the famous midrash about Avraham seeing a house lit up and saying that the house must have someone watching over it. He similarly concluded that this world must also have a Being guiding it. At that point, God revealed Himself to Avraham and said that He is the owner of the world.
R. Yitzchak Sorotzkin (Gevuras Yitzchak, no. 12) quotes the Brisker Rav as asking what purpose the last part of the midrash serves. Avraham had already deduced God's existence, so why did God have to reveal Himself to him?
The Brisker Rav quotes the Beis Ha-Levi (end of Bo, p. 27b in the old editions) who writes:
Whatever is clarified to a person through proofs is called knowledge and not belief. We are commanded to believe, but that is on matters that the mind cannot prove. As long as someone only accepts that which he can prove, he has no part of the command to believe.Avraham could prove the existence of God but he could not deduce the extent of God's unfathomable being. Thus, after Avraham proved that God existed, God appeared to him in prophecy and informed him that God is an infinite and all-powerful being.