That is akin to "the law already protects the innocent citizen, no need for a lawyer to dip his beak into the citizen's money bag."
A law is just some sheet of paper, you need people to actually enforce it.
The difference being that aside from a small retainer fee, a lawyer doesn't charge you unless he actually does something. A Union, meanwhile, is more than happy to siphon your paycheck for decades without doing a damn thing.
Also, if there's a law protecting your job, you don't need a Union to enforce it, just a lawyer. If you have a Union, plus a law protecting your job, now you pay the Union, then they go hire the lawyers on top of that. Unions are primarily useful in situations where there is NOT a law protecting the workers.
Actually most if not all of the laws protecting workers came as a DIRECT RESULT of unions.