They are also very effective in removing baked on bug guts and tree sap.
Rub your hand along your hood. If you feel bumps in your paint from bugs, tree sap, road grime etc, use a quality clay bar to remove them, then apply whatever wax or polish you choose.
I can vouch for what a claybar can do. Sometime over the winter my expedition got covered everywhere with some kind of an overspray that looked like surface rust (scared the hell outta me the first time i washed it). Granted it took me a good three hours, but every last speck of it came right off when i detailed my truck last thursday. If nothing else, they're worth their weight in gold for just gettin bugs off the front end and any crap your wheels kick up on the fenders.
it prepares your paint for fresh polish and wax. it will get all the the impurities out of your clearcoat to ensure and even coating of wax and polish, that will help give your paint the ultimate "wet" look. you don't want to wax the dirt embedded in your paint do you???