Originally Posted by
mdilthey
On my bike, first and foremost it's about clearance. Front tire measures as a 2.8, rear tire is just slightly wider than a 2.4 (wide rim).
Also, though, a slightly wider tire in the front is good because of traction. If your rear tire skids out, you'll probably be fine. If your front tire skids out, you're almost certainly going to crash.
That and also for a rigid frame on rough surfaces a bit more cushion can be nice. Back when I was racing mountain bike and dinosaurs roamed the earth, most racers that I knew used a slightly fatter tire on the front even where clearance wasn't an issue.