Why did Dracule Mihawk become a Warlord/Shichibukai? If we see the list of Shichibukai, almost all of them have some reason to become Shichibukai. 

Boa Hancock became shichibukai to protect Amazon Lily from marine. Gekko Morai, Doflamingo, Crocodile etc had their black work or evil deeds, which needed to be protected from marine. 

But I don't remember anything about the Mihawk. He is alone but strong, so he don't need to fear from marine. 

What is his real intention to become warlord?
Seeing how there is no motivation stated in the canon for Mihawk becoming a Shichibukai, I assume his reason to ally with the Government was to be able to access assets that would be reserved for members of the World Government only, such as the Tarai Current or passing from Paradise to New World via Mariejois, thus allowing him more freedom of movement. Mihawk reminds me of the wandering swordsman, so more freedom of movement sounds like something he might be interested in