to check whether it is sound.
for speed (quicker than tacking up etc)
because the rider's back isn't right or something.
to teach it to stretch down.
because it has a sore back from the saddle or something, and needs exercise without the weight.
to check whether it is a lunatic today, before you get on!
to get rid of the bucks, if it is in loony mood today...
to check whether one-sidedness is coming from the rider or from the horse, e.g. in canter.
pure flipping laziness, in some cases, imho! *awaits incoming brickbats*
if your school needs ploughing, it's very good for that.
btw can you tell that i hate lungeing, and only do it with the babies!
As well as above mentioned - i use it look at how the horse is moving; perhaps i am a bit thick but i find it helps me to ride her better if i can see her transitions and paces on the lunge. Some winter evenings as well when it is and wet, and i've had a hard day (AAAHHH!) i am inclined to lunge rather than ride in the school.