Motion Sensor Light -Gradual vs. Instant On

I recently installed a third double floodlight unit under an eave in my backyard. All three of the units have the standard 180 degree motion sensor capability. This last unit, is a Zenith Secure Home model, just like the other two.



However, this last unit seems to have a slightly different process, when the motion sensor detects movement. On the newest unit, the floodlights gradually illuminate, instead of illuminating to full brightness immediately.

I assume this gradual illumination is controlled through some sort of variable resistor in the circuitry. But I wondered why this gradual illumination capability was incorporated into the unit. Do you figure this is to try to protect the floodlight filaments, since most bulbs seems to fail when they are initially illuminated?