To be more verbose. All-in-one systems typically have small reservoirs at one end of the radiator. Used by the vendor for filling purposes.
But no, you don't actually need one in a water cooling system. They are convenient for priming and filling a system. The only real benefits they offer is a place for air bubbles to work themselves to, having a reserve of liquid to guard against minor evaporation loss, and adding a little thermal mass to a system. One could also argue for some hydrodynamic properties some reservoirs offer for increasing flow smoothness, and thus increasing flow rate.