by randomness in spells, i mean that the effects might be stronger or weaker occasionaly, or they may have a shorter or longer duration
imagine the suspense of sneaking into a fortress with powerfull guards, you cannot tell if they see you- so you just have to hope they dont, as you get to a certain point, you must go invisible and walk across the open in the midst of hundreds of guards. you better hope your invisibility doesnt fail on you.
imagine that you get to the end of the open bit, but your invisibility fails. the guards turn and charge at you. you cant hold that many guards and you need to do your job quickly. the best solution would be to summon a pair of powerfull dremora to cover your escape. but your summoning skill is low and other factors mean that you have a 10% chance of the spell failing, and another 10% chance that you wont bound the dremora properly, so that they attack you instead. you need to take the risk.
your target is infront of you after you escape,you are hidden and you draw your dagger. there is a 50% chance that you will kill him instantly, or atleast wound him enough to finish him of with ease. the other 50% means that he will turn around, grab your dagger and do some crazy judo throw with the possibility of executing you whilst you are down.
you get my drift?