A subject briefly touched on above:
On many (but not all modern vehicles) the front tires do most of the braking. (remember, on some vehicles, with the advent of the latest gen ABS, rear tires can share the light braking loads evenly with the fronts, at least initially, for better braking feel)
So, front tires, if left on "too long" and when hard braking is used, can often develop asymmetrically worn tread blocks. Rub your hand "back to front" over the top of the tire. Then the other way.
Rotating tires helps deal with this type of wear. And the related tire noise.