I ride a fatbike year round that has a dropper post. In the winter I primarily use it to use the bike to sit on - with the post down I can put my feet on the ground while chatting, waiting for someone etc. Sometimes used to drop my feet to the ground to use as outriggers when I encounter a stretch of ice.
In the summer its primary use is for lowering my CoG when taking sharp turns on steep hills. Sort of a combined super-tuck, stability deal. Ripping down a mountain highway at 70km/h getting ready for the next switchback is something I *would not do* on my fatbike without a dropper post.
The only other time I can remember using it was for manual/wheelie practice - with legs as short as mine there isn't a lot of difference between sitting and standing so to get behind the seat I need to drop it. LMAO