Many times after a wash, shampoo and water simply will not be strong enough to remove things like paint overspray, sap, tar, caked on rubber, bug splatter, rail dust and other heavy contamination. The purpose of claying is to remove that contamination still left behind from a wash process. Sometimes, the contamination remaining on the paint is not visible to the naked eye, but if you were to run your hand over the paint, it would feel bumpy or rough to the touch. This is often a great test to see if you can benefit from a clay bar, gently glide your fingers over a wet area of your paint and if it is not as smooth as glass, you can benefit from a clay bar.
New cars can also benefit from the claying process because the vehicle is exposed during transportation and delivery, not to mention, you don’t know how long your “new” car has been sitting on a lot. Things like rail dust, industrial fallout, acid rain are all things that new cars are subject to and claying could be your answer to removing them. Lastly, claying helps enhance the vehicle's finish when polished and/or waxed.