What is the purpose of fenders?

DH and I were talking about this last night and while we know that they keep the water from splashing off the tires/road, why do we see them listed as 'essential' when commuting? I mean, if it's raining, you are going to get wet...whether you have fenders or not, right? Or do they really work that well?

I can see how they would be good for touring, where you could be out in the rain for hours on end and where a wet pack would REALLY suck...but for commuting?

Tell me why you love your fenders (or why you hate them?)!

Thanks!