DSB give a place for anaerobic respiration to occur which emlininate trates, turns them into N2 gas, completely inert.
a remote bed (seperate from sump or tank) is best incase any noxiousness does build..
a 10 gal hooked up to the tank, with it's own pump, full of sand would be interesting to say the least...
they aren't by any means necisscary, i have them in y tanks but i like jawfish and gobies and such so I need them..
a good canister or a regular sump wors great filtration wise but then you'd be doing WC's for trate removal