Radiators have a dual heating effect: convection and radiant heat. How does that work? The oil in a radiator heats up and flows around. This warm liquid in turn heats up the air, creating convection, just like a convector heater.
As the hot oil flows through the fins, they also radiate heat. The more fins there are, the more surface area there is to emit this radiant heat. This makes a radiator like heater very efficient to use, and ideal for heating rooms just a little extra electrically.