If you provide health care, you always have power over that person. Less so if they are relatively healthy - but any condition, theirs or a family members, means that the person has no real choice but to do enough good work to keep the job. Even if they hate the company. Even if you treat them poorly. They still must work for you. This is even more true if you have hundreds of people that will replace them and your health coverage is good enough - or at least, better than the opposition.

To a lesser degree, the same goes with vacation time and other benefits. At least in the US, anyway. This is why having some of this stuff coded into law and decoupled from employment takes some power away from employers.