I don't know of anything called "wheel dressing". You may be thinking of either a wheel cleaner or a tire dressing.
A tire dressing is a liquid or gel that you apply to the tires after washing them, which does several things for them:
*Keeps the sidewalls from drying out and cracking. The dressing serves as a sacrificial barrier, which protects the sidewall from exposure to UV rays and ozone, which will deteriorate the rubber over time.
*Keeps the sidewalls black, and anywhere from glossy to matte, depending on the product used.
*Makes the sidewalls repel water, and makes them easier to clean/dry.
A wheel cleaner is a strong cleaner meant to loosen brake dust and road grime from wheels. A typical car soap solution usually isn't strong enough for the initial cleaning of dirty wheels. Once you use a wheel cleaner and polish/seal the wheels, a car wash solution is typically plenty strong to clean them.