I use a walking foot for all the reasons given by the others. I use it for matching plaids and stripes in clothing construction, binding, straight line quilting, and for piecing long strips of fabric. It's a pain to put on and take off as I have to change not only the foot, but the post that holds it.
My next machine will be a Pfaff with the built capability.
They do wear out eventually at least the one for my Babylock did. The machine isn't that old a couple of years or so. My son-in-law to be put it back together, but every time I hit an extra think part of a binding or wallet it comes apart again.