The lapping / the sun cut of a watch case is a special finishing of the watch case. Over time, the grinding pattern of your wristwatch will wear out or there may be scratches in the grinding pattern that break the symmetry of the decorative cut. Straight edges are rounded off by incorrect polishing or heavy use.
Lapping your watch renews the grinding pattern. Broken and unclean edges can be corrected so that the original shape of the watch case is restored.