Warlords get a bunch of benefits for helping the marines. Not that it matters, since the warlord system is broken and most of them do their own things.
He's good friends with Sengoku. Garp had a bunch of achievements when he was younger.So I have a One Piece question: How is Garp allowed to hold a high position in the Navy when his son is the revolutionary leader and his grandson is one of the most notorious pirates in the world?