I use them on all my goggles for 2 reason:
1: I wear glasses, and thermal lenses don't do crap for your glasses fogging.
2: I sweat like a pig. If you set the fan to pull air out of the goggles, then it blows a nice cooling blast right onto your forehead. I used to get sweat dripping down into my eyes all the time, but the fan helped to limit that to only the hottest days.
They can be a little loud, but the switch is easy to get to while you are playing, so I just turn it on and off as I want/need it.