It's a tool for intimidating workers. Non-competes may make a worker afraid to leave a job for fear of being unable to work in their industry. It may also make a worker afraid to take specific positions for fear of retaliation by a former employer.

The key is, non-competes can accomplish these things even if they're not legally defensible. Your average worker doesn't have the legal knowledge to know if a given non-compete is actually likely to hold up, and will often assume that they signed a legally valid contract. Most workers can't afford to go to court, so the threat of a lawsuit can be used to bully them into submission.