You'll get the raise because your union contract specifies the raises that people get. When I was in a union, we knew exactly how much our raises would be for each year of the contract. The contract defines all the work rules for those in the union. It may define work hours, overtime rules, benefits, policies for handling disputes, etc.
You pay union dues which are probably automatically deducted from your check (if you're in a closed shop, which means everyone is required to join the union).
The benefits are that you are protected from improper treatment by management. If you are treated unfairly, the union will step in to defend you, to the extent that the contract allows.
I have seen both sides of unions (I was a member, my father was a chief steward) and they can be very useful in a company where management is abusive and unfair. On the other hand, in a company that has good management but uncooperative union officials, a union can be counterproductive. Much depends on the people involved on both sides.
When I was outproducing my union coworkers by about 50%, yet we all got the exact same raise, it wasn't doing me any good. I had no chance to make more money by doing a better job. I was not allowed to run or connect a network cable - I had to call someone else (the electrician) who would come by a week later and intentionally take an hour to do a five-minute job. That person would bring an apprentice who was the electrician's personal valet - fetching coffee, tools, drilling holes, etc. It cost about $100 to do what I could have done in literally five minutes. I ended up coming in on Saturdays to run cables myself just to get things done. So that was an example of a bad union situation.
My wife used to work at a place that was a complete sweatshop. Abusive management, people getting fired arbitrarily, having to work overtime with no notice, etc. They needed and would have benefited from a union. They tried to organize one but it failed because the management scared people into thinking they would lose their jobs if the union was voted in. Didn't really matter much because in less than four years the company went under.
It all depends. Can be good or bad.
