Hi: I would be lost without the built-in walking foot on my Pfaff. It's engaged for about 90% of my sewing with the exception of the "big guy on the bottom" sewing. You can do the experiment: cut two stips of fabric the same length (about 12-15") and sew a seam without a walking foot. You will notice that the upper layer ends up "longer" than the underlayer. Now try this with the walking foot - both are the same. With the walking foot engaged you have no more problems getting both layers to match, and this is great for plaids and stripes that you will no longer fear sewing!
What's "big guy on the bottom" sewing? Setting in sleeves and sewing princess seams, where the longer layer of fabric is on the bottom and the shorter on the top. Disengaging or the absence of the walking foot helps to ease the sleeve or curved seams together.