Are you asking why the mob was attracted to unions or how they used them?
Unions used to be a much larger section of the private marketplace than they are currently. Mob control over or influence in a union means at the very minimum that mob associates or members can get no show jobs, legal income and legitimate health care coverage/pensions. But that's pocket change. Union control means votes which means political influence. And if you control the union not only do you get to extort the working man (union and non union) in a variety of ways , you can do the same thing (extortion/racketeering) to businesses and even municipalities or states. And if you're smart enough and entrepreneurial enough there's no end to the games you can play with pension fund loans and withdrawals. You can throw work to favored companies-i.e those owned by your in-laws or other relatives. You have an inside track to steal whatever goods the union happens to produce. You have a captive market for loansharking and gambling. Depending on the type of union you might even have a reserve force of muscle that you can use for jobs. Back when unions were common and the Mob was strong, there were tons of ways to make money.