Shower faucets often have large holes cut through the tiles to allow the plumbing through. These are then covered up by escutcheon plates, those decorative plates behind the handles. But these plates are not water proof and often water rolls down the tiles, behind the cover plates and into the wall. This is often the cause of loose tiles below the faucets, or deteriorating grout.
On this segment of my TV show we showed how to use Plumber's Putty, or even caulking to seal between the plumbing fixture and the tiles behind the decorative cover plate. Plumber's Putty may crack with time but is easier to remove than caulking when you need to do some plumbing work on the valve itself.