Deep sand beds help control nitrates and phosphates for a limited amount of time before you have to change them out. Says 180 pounds for a 4 inch sand bed on a 48x18" tank. Most people say a DSB is worth it. Plus it will allow you to keep jawfish.
just my take on things as a DSB user for many years...there's loads more info out there though if you hunt around... Just do a search for 'Deep Sand Bed Reef Aquarium'