Maximum surface contact is the goal with lapped rings. I've never noticed a different when fielding the rifle, but when initially setting one up and getting it mounted, lapped rings let the scope move smoother than unlapped rings for sure. At least that's what I've experienced. I don't purposefully buy lapped rings, if they come to me lapped, fine. I won't lap them either. YMMV.
Not just that. Alignment of the holes through the rings a goal, so when the rings are tightened they do not bind the scope.
