Controlling a union is an extremely lucrative business. Corrupted union officials can easily put the squeeze on companies and require payments for "labor peace".
If the Mafia has influence over a labor union, it can allow a Mafia-controlled business to operate without a union contract, while forcing unions to enforce the union contract against non-Mafia controlled businesses. It´s all about creating a "monopoly" in the field where ever the Mafia is involved. Or trying to gain an upper hand against the competitors.

But there are legitimate ways to use the union as well.
Placing a Mafia member in a legitimate job in the union; placing a Mafia member in an official position in the union; or having a member in a "ghost payroll," or "no-show" job on a construction site for example.
A "no-show" job is when a Mafia member recieves payment, salary,
without actually doing any work, not showing up at work at all.

But let´s not forget the Big-Daddy-o way of extracting financial means from unions and it´s members; controlling the various worker´s pension funds connected to big, powerful, strong in numbers unions.
A pension fund is supposed to pay out to retirees and/or to former workers who suffered disabilities connected to their work. The trick, however, is that the pay outs always are held far below than what it should be.
Complainers who are victimized often find several technicalities in the fund´s regulations that give the fund the right to withhold parts of the payments. The fund uses loopholes to keep as much money as possible.
The Mafia often has control over these funds and uses the money in investments. For example, a lot of money from Teamster´s pension funds were used for hotel and casino investments in Las Vegas or other real estate investments. And money from the funds are used as loans to Mafia members or its cronies and put into fully legit enterprises. But of course, when it´s time to repay the loan, the enterprise file bankruptcy or is out of business.