Why Would Arjuna Need The Flag Of Hanuman On His Chariot

“O King, at that time Arjuna, the son of Pandu, who was seated in his chariot, his flag marked with Hanuman, took up his bow and prepared to shoot his arrows, looking at the sons of Dhritarashtra. O King, Arjuna then spoke to Hrishikesha [Krishna] these words:” (Bhagavad-gita, 1.20)