I was told that they started offering the combo prior to the On/Off switches being widely available. If bound to a canopy module, there is no benefit of using the Dimmer over the On/Off switch in terms of operation, but the On/Off switch has the added benefit of being able to kill power to the canopy module if needed when the air gap tab is pulled.
Again, this is because the Dimmer is only rated for resistive loads, and the On/Off switch is rated for inductive and resistive loads, therefore when using the On/Off switch you can attach the wiring to the canopy module to the Load portion of the switch, while when using the Dimmer you need to connect it to the Live portion and leave the Load terminal not connected to anything.