We know that Lot accompanied Avraham in all his journeys, so why does the Torah see fit to mention it here?
Rashi: To teach us that it was precisely because Lot accompanied Avraham that he merited to become wealthy. 1
This also seems to be what Targum Yonasan is saying.
רש"י: מי גרם...הליכתו עם אברם: מנין שאברהם הוא הגורם?
גור אריה: אם לא כך, "ההולך את אברם" מיותר.