It doesn't change the thickness, but it prevents the spacer from seating flat against the front of the cam journal if its installed with the chamfer pointing forward instead of toward the rear. The chamfer is on the inside of the spacer, and its machined there to allow clearance for the radius that is machined on the front of the cam snout where the snout meets the cam journal. If you install the spacer on the snout backwards, the spacer can't seat flush against the face of the journal because there is no clearance for the radius that is present. And since there is no clearance if its installed backwards, the cam gear stops short of where it needs to be and has too much end play.
