Why do Printing Shops need to put flyers on cars? -

They do it as an ad and since they are printing shops they can both demo their product and basically print it for free.

exactly what Diogenes said.Also it is a cheap form of advertisement for them.However, in my personal opinion it is a waste of resources and is not as effective as they might believe. Research has shown that the average person needs to be exposed to a product or printed ad a minimum of 3-5 times before they will even read it and 5-8 before they will remember it or use it. And unless you park in the same spot every day and receive the same ad every time it is very hard to accomplish that