The awesomeness of having the Merc with a Mouth and the baddest bounty hunter of them all teaming up shouldn't need any explanation. But, I'm going to anyway!
The idea is that they are both mercs of some sort already, working for money and taking jobs, allowing for both single-issue content or long multi-issue arcs. Boba Fett plays the "straight man" archetype with Deadpool as the comedic relief and more light-hearted character. This allows you to play their nature against one another in an interesting dynamic where Wade can poke some fun at Boba's grim demeanor and generate some truly comedic panels and issues.
I personally feel the following that both of these characters have in their respective fanbases is huge and a sort of "buddy cop" style series would make literally truck-loads of money.