I've always believed you need to see what customers drive up in, because it can often give you clues on which direction to take them. It’s not to see if they drive an expensive or low-end car, because that will make you assume something and often in the wrong direction.
A person in a rich looking car may just need builder grade for a rental home, while a couple in an old beater might mean they are frugal but want something nice for the inside of their home.