Forced airflow is several times more effective than relying on convection. Remember that hot air rises, so if the room was narrow, removing the shelf would indeed be sufficient. However, if the end of the room is quite far, it will take a quite a little while for heat to circulate.

The fan speeds up the process by sucking a lot more cold air in through the radiator, forcing it against the warm metal and blowing it out into the room. (Think why the heatsinks in your computer have fans.)

The shelf may act as a way of ensuring air blows through the entire depth of the radiator, but I doubt it has any significant effect either way.