His business with Roth was probably a potpourri of illegal activity most of which focused on the Cuban casinos. Since it was international and since it involved a partnership with a government and since it involved potentially huge profits, it was very important. Although it's not expressed in the movie, the Cuba connection was probably also important to drug trafficking.

No, I don't think it had anything to do with Michael's attempts to go legit which I don't think Michael had any intention to do.

All Mafia dons are supicious of everyone else in the business. There is only faux trust - there are only working relationships that end when it is convenient to end them. He had to work with Roth to achieve what he wanted to achieve.