What annoys me when I ask such things to Muslims they tell me only God knows - he has a greater purpose than we know or can understand - he has just revealed what we need to know!
I find it funny somehow... Allow me to play around a bit... (trying to get it!)
So, after death and/or judgment, we will receive more answers. But, if we already weren't capable of understanding the answers (because of our human limitation), then the answers must come along with some change in our being. I mean, only if we cease being the limited humans we are, and gain somehow a new level of being - we would be capable of receiving/understanding the answer.
What does that make us? Angels/Satan?
If it makes us angels, bad news for who believes in Satan: angels have freedom of choice too, and can be held responsible for their choices in front of God, etc, so even if they have a better understanding than us, they still can - and do - the same as us. Not exactly a promotion, then, heh. And still can't explain how come they do something wrong then, if they TRULY understand.
If you think angels are perfect (so no Satan), you're in for more trouble. Say they understood the point of it, and they're happy in eternal grace praising God. Well, then our ultimate purpose in this life is to be worthy of being allowed to become what today we might call "dumb sheep". Not even mentioning the fact that you kinda have too many "perfect" beings here. *headscratch*