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. Gur Aryeh writes that Rashi derives this from the extra phrase "who was travelling with Avraham" (for this has been mentioned as recently as 13:1).
רש"י: מי גרם...הליכתו עם אברם: מנין שאברהם הוא הגורם?
גור אריה: אם לא כך, "ההולך את אברם" מיותר.