A refugium's task is more to give the tiny macro critters a "refuge" away from fish and other creatures that feed on them. To give the pods/worms a chance to breed - which in turn feeds the corals, fish, etc....
Most people do not have this seperately, but include a refugium in their sump as part of the DSB - as only a refugium require macro algae to support the breeding of pods - this also happens in DSB's - therefor it's a logical option to combine the two.
With Nano tanks it's slightly more difficult - as no sump = no DSB = no refugium - therefor adding a small hang-on refugium makes sense....