Why? Ribs expand and contract with the weather and season. A chinrest that is tightened when the ribs shrink will cause the ribs to warp when the weather changes and they try to expand again. This is especially a problem with chinrests that clamp to the side of the endblock. Chinrests that clamp over the tailpiece are superior because they clamp to the endblock and are much less likely to deform the ribs.
The chinrest can also come into contact with the tailpiece and cause buzzes. Check for clearance.