If you've formed an LLC, most states require you to name a registered agent, a person or company designated to receive legal and official mail (like a lawsuit notice or state compliance letter) on behalf of your business at a public address.
You can usually serve as your own registered agent if you have a physical address in your state of formation (not a P.O. box) and are consistently available during business hours. Many freelancers do exactly this to avoid an extra annual fee, especially early on.
A commercial registered agent service (typically $50-$150/year) makes more sense if you work from home and don't want your home address on public record, if you travel frequently and can't guarantee availability, or if you've formed your LLC in a state where you don't actually live. It's a small, optional expense, but one worth understanding rather than defaulting into blindly when you form your LLC. See the broader LLC guide for more on formation basics.