Originally Posted by bostontrevor
There are a couple of reasons. 1) You're carrying someone's property and you want to make sure it doesn't go anywhere on you. II) It can be wider than what you might carry on the back because you can see it to maneuver it. c) It's actually better to weight up the front wheel for handling because bicycles tend to be too rear heavy to begin with. Adding more weight makes it handle squirrelly and can even make it buck while you're trying to mount up if you put too much (say 20 kg) on the back.
Not talking about front versus back racks. I've seen some euro bikes where the rack is in the front, suported by an extension from the top/down tubes, not the front wheel. This gives you the advantages of the front rack, without having to actually wrestle all that weight around whenever you move the front wheel.