Car shampoo, containing carnauba wax, boosts your car's gloss by adding a layer of protection and shine during the cleaning stage. If you have a layer of carnauba paste wax or hybrid wax already applied to your car's paintwork, your car wax shampoo will increase its longevity, so those 2 months of protection can easily become 3 or 4. The shampoo achieves this by maintaining the very top layer of hydrophobic properties given by the wax, filling in those small grooves created over time.
Even if you have no layers of protection on your car paintwork, it is still better to use a car shampoo with a wax or gloss boosting property. Not only will this make the car easier to clean, but will also leave behind a small layer of protection for the coming weeks. Autoglym's award-winning Bodywork Shampoo Conditioner is an example of one of the best shampoos, leaving behind rust inhibitors and a hydrophobic barrier all in one.
For cars protected in a ceramic sealant, it would be best to use a pH-neutral shampoo or sealant friendly car shampoo. The wax properties in the other shampoos can clog the pores of the sealant and make them less effective overtime.