A rewetting agent is generally applied after etching with a mineral based acidic system e.g. H3PO4  -- these agents were developed with the intention(s) of rehydrating the collapsed collagen so that the adhesive primer polymer could penetrate thru the collapsed collagen fibres.  In addition, some manufacterers claim that their rewetting agent also serves as a surfactant that is supposed to increase the spreading effect of the adhesive primers over the restorative interface.  From a chemical standpoint -- the X-PUR-Crystal also is a surfactant that reduces bubble formation in preparation for the taking of an impression as well as to spread the primer system.  Of course, there is NO reduction of the final bond strength when using X-PUR Crystal -- Dr. Cox.