Why do baboons groom each other?

Over and above the basic benefit of removing parasites, debris and dead skin from a baboon’s coat, grooming is a primary form of bonding, and forming and maintaining friendships. For the baboon being groomed, it’s a highly pleasurable, comforting and relaxing experience which lowers their levels of stress. 

It’s also a form of currency that a lower-ranking individual can trade with to get close to another baboon, who may in the future become a protector, mate, ally or friend.