Of course, unionising doesn't necessarily mean asking for higher wages - that's one of a number of things unions traditionally do. It can also be used to bargain for increased safety, increased job security (making it harder/more expensive to fire people), better-overseen disciplinary procedures, and provides the guarantee that there'll be someone at the table with any employee when the company starts picking on them.

The idea that unions can only be used to bargain for wages is propaganda in its own right.