The AHG(RH) certification does not have any legal meaning; instead, it is a peer-reviewed certification that shows that an herbalist has achieved a certain level of proficiency, as evaluated by their herbal peers. It is meant to provide a standard that represents an herbalists training, level of expertise, and skill.
To apply for the Registered Herbalist designation from the American Herbalists Guild you will need an advanced background in herbalism, with at least 800 study hours and 400 clinical hours. Some herbalists choose not to become registered herbalists, even if they do meet the requirements and others feel that it is important because it gives clients a level of comfort as to your abilities.
It is not necessary to be an RH to see clients. It really is a personal decision whether you feel you want to show that you meet the AHG(RH) requirements. Achieving the requirements for RH would also be a way to guide your career development.
You can review the AHG criteria here: http://www.americanherbalistsguild.com/sites/default/files/criteria_and_instructions_for_ahg_application.pdf