It's not a question of simply losing your domain. If you were to have legal action taken against you, they could seize any of your assets, including your earnings and other domains. With a business entity, you're making sure that they can ONLY come after your business assets - in other words, only what's in your business bank account, not your house, car, etc. Without that, if you get sued, they can come after all of your personal assets.
I can't speak for an S-Corp, but at least with an LLC, you are merely a member and representative of the company. You and the company are two separate entities, thus ensuring that you're protected in the eyes of the law.