Thickness of PV wafers is measured for two reasons.  First, it is measured to insure specfication, both in total and the variation within a wafer (TTV).  Second, it is measured because resistivity measurement, using the eddy current technique, needs thickness information to report the bulk resistivity of the sample properly.

Of course thickness is also measured by wafer reclaimers as it may determine whether a wafer can be reclaimed or used as scrap silicon.