Using registered agent address as mailing address
Registered Agent serves a specific function - to be "there" in case your company is being sued and is served with court papers. That means if a server of process is dispatched to the registered address to serve your company they will be able to submit those papers in person to a registered agent representative at the location during business hours.
Some states would also send certain documents, such as annual report notices, to the registered agent, and the registered agent would then forward those to you.
With that being said, registered agent is not a mail forwarder. Some registered agents offer such service for additional price, however I believe its always cheaper to just rent a UPS store box and ave them forward your mail to you.