For me it is mostly freedom. Employment contracts usually go into "all your code belongs to us", "we own every piece of thought you ever had", "if you want to m

For me it is mostly freedom. Employment contracts usually go into "all your code belongs to us", "we own every piece of thought you ever had", "if you want to make PR to a project you like first write an email to your manager with approval, then you have to forward approval to owner of project". If I want to make a side project I don't have to worry there will be any issue with copyright or god knows what. Because I am my own company and single customer cannot include in contract that I cannot do any other work.