Most states have laws which require this kind of registration before funds can be held or transferred under the name in a bank account. Although most counties and states will not register two identical or highly similar business names, others will, so you can't rely on an assumed-name registration as proof that no one else in the county or the state is doing business under the name. Your date of first use is usually kept on file, and registrations must be periodically renewed. Because the right to use ultimately comes from being first in time, you should check existing registrations in the county before selecting a company name, to determine if there are already other users. State registration eliminates the need to give notice to others who may begin using a similar mark or name after you.
Protection extends to the state borders.
Also, states usually allow the registration of business and trade names as trademarks, while the federal system does not. Even prior common-law users cannot challenge a mark once it becomes incontestable under state law. And, state fees are typically low. I have a corporation and it has an assumed name. If you are already a corporation for liability purposes, name certification is a bonus.