If you sell a product or service, you probably already have a trademark (at least a common law trademark). And if you have a trademark, you need to protect that trademark by registering it with the US Patent and Trademark Office.
Filing a trademark registration at the USPTO provides distinct advantages over relying on your common law trademark. Crucially, filing earlier than others that want to use the same trademark will give you priority over them. A timely filed trademark registration can make the difference between having the ability to stop others from using your trademark, and not even being able to expand the use of your trademark to other geographic areas.
So while trademark registration is not required, it is strongly recommended and can prevent significant expenses and headaches in the future.
Learn more from our Introduction to Trademarks.