In addition to what the other commenter said it also prevents the employer from obligating you to stay, for instance, by including a provision in your contract that requires notice of your intention cease employment.

It’s worth pointing out that there are no exceptions for anything that would violate state or federal law. Even here you can’t fire somebody for their gender or sexual orientation etc. However, functionally that means the burden of proof is on you to show that you were let go for illegal cause, which can be next to impossible if you aren’t vigilant.