You're either uninformed or have been blinded by the anti union propaganda. There's a severe power imbalance between a company and an individual worker. Even if you're a highly skilled worker and have, according to the companies that push anti union propaganda, more bargaining power compared to the average worker you're still very likely to get the short end of the stick.

But collectively workers have power. Why don't we have companies that consist solely of shareholders and c suites? Because they produce no value without the workers. The value is generated by the workers and without the workers the company simply won't do anything. But it's hard to coordinate all workers to collectively use their power.

And that's the purpose of unions, to represent the collective bargaining power of the workers. Unions are extremely powerful and that's why companies love to lobby anti union propaganda. If you're a worker you should always be in favor of unions (unless you go even more left in your political views and see unions as a compromise to maintain the capitalist system).