Its used as reassurance to make sure your being compleatly honest. If you sware on the boys and a bro finds out that you lied, your not concidered a true friend. One is never supposed to be dishonest while swaring on the boys, its just something you don't do.
bro 1: Did you see my x-girlfriend with Alec last night at that party you went to?
bro 2: Nah man she was solo.
bro 1: On the boys?
bro 2: Alright alright alright I'll tell you verything I saw if you sware on the boys you wont tell Alec.