Not necessary, but always a nice to have. The whole point of adding the fan to my keezer was to evenly distribute the temperature. As the compressor kicks on to cool the contents, the fan turns on to create flow throughout the keezer. Pushing the cooler air from the bottom towards the top, thus moving the warmer air and eventually equalizing the temperature.
To note, my thermocouple was hanging freely down the side and is resting on top of the compressor hump (shelf) on one side. I wanted to move the air around when the compressor is on so that the temp would be equal from top to bottom. That helped me to keep my dispensing pressure uniform. As I was trying to figure out the perfect CO2 pressure for dispensing the beer, I noticed that the fluctuation in temperature was doing weird things with the amount of foam for each pint poured. Once I added the fan, things simmered down. I also didn't want the contents of the kegs (towards the bottom) to be cold but the beer in the lines and at the taps to be warmer.
Hope this helps. Cheers!