There's an actual cost the company is trying to avoid though, and that actual cost will have to go somewhere eventually. Either it will actually result in companies having higher overhead to maintain their business, or the employees who used to have non-competes having to pay for some of the firm's business generation one way or another.

Reminder that the question was "what are good reasons...", which suggests that not everyone knows why the company wants them, unless you assume all companies are inherently evil which is not reasonable. They invest in their brand. Employees get the benefit of that to help establish their own brand. And then they can take that brand elsewhere to the detriment of the original investor.