To avoid damaging the blade or the hook recoil dampers are frequently fitted to self retracting tapes. The mechanism absorbs energy when the blade is recoiled into the casing and slows it down. This reduces the shock effect on both the hook and the rivets that hold the hook on to the blade.
All the pocket tapes from Carl Kammerling International (except the 3m ST tape) feature recoil dampers.
The clip provided on the side of many pocket tapes is a belt clip to allow you to carry it easily. The clip is usually screwed into place.