Without a a union there's a power imbalance between employer and employee. People say it's all based on mutually agreed contracts. In reality, you are probably much more dependent on your job than the company is on you. That's because they may lose something like 1/1000 of their employees when they fire you, but you lose 100% of your jobs[1].

Unions are a way correct this balance. Employees band together, so that failure to reach an agreement is as painful for the employer as it is for the employee, i. e. (in the worst case) work stops and nobody gets paid.

[1]: If you doubt me, try getting Google to come in for 6 interviews and make your future manager do a whiteboard exam.