Crocodile had the idea, because he was the leader of a bounty hunter organization
but like before, he doesn't like being the head of it, he likes hiding his face, so this time to make sure he wouldn't be the one everyone would be gunning for, he called up Buggy to be the face of the organization
then, by calling Mihawk to join them, they get more power and respect, and Mihawk gets to chill without having to deal with shitty marines all the time, so he accepts
Crocodile is the brain, Buggy is the face and Mihawk is the muscle / sword
Best. Fucking. Yonkou. Crew.