They use force to threaten corps into submitting to their demands. The belief goes that corps would just "exploit" workers if they didn't exist. But such a thing is fallacious when you actually think it through.

If a corporation decided to not pay a wage that was worth a worker's time as the saying goes that we need someone to come in and arbitrate on our behalf in order to raise that pay. But if such a thing was true then said corporation would not be able to hire anyone because they aren't the only corporation to work for therefore people would just go work somewhere else.

The actual fallacy in this notion is that someone somewhere can deem what would be considered a livable wage and then apply that equally across all of a sector of industry. Instead of letting each individual person decide what wage they should work for we demand and deem a certain level of some arbitrary number as the reasoning for why we need This union to come in and protect us from the greedy capitalists.