The good part is that the employer is not afraid of hiring someone. I'm from somewhere where employment is a serious commitment - after some time of employment, it's very, very hard to let someone go.

The consequence is that employers are deadly afraid of getting stuck with someone who is not a good worker, basically forever. It means that salaries stay low, because if you raise a salary you can never bring it down again, and you can't get rid of that worker. It also means that some workers just stop giving a fuck as soon as they get "tenure" and can't be let go anymore.

At will employment has a lot of downsides for the worker, but it also means that no one is afraid of hiring and so that you can easily get a job when you need one.

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.