It varies per game, but my most common use is to replace the thumbstick clicks. I don’t like how it feels to click in the thumb sticks, feels like I’m adding extra wear and tear to them.
In first person games, I’ll often bind jump, sprint, reload, and melee to the back buttons so I can move and aim while performing those actions.
The utility is proof that he’s killed 100 of Saul’s enemies, and Saul is possibly hoping that David will get himself killed in battle.