Having a registered agent is only mandatory for foreign entities, but it is still optional for domestic businesses. Most domestic businesses utilize their own physical place of business, if they have one, as the “registered agent” location.
In the case of a foreign entity, they typically do not have a physical address in their state of incorporation, but they need somewhere for mail and service of process to be received. The idea is that the registered agent will be responsible and discreet when handling the private and personal documents involving the entity.