It prevents people with lots of dirty money from paying expensive-but-unregistered-goods (i.e. not cars and houses) with cash from law abiding citizen; if they want the expensive stuff, they'll have to either find some other dishonest businessman to sell it to them (might be hard, depending on what you're buying, and increases the risk of being caught) or deposit the money in a bank to pay for it electronically (which will raise flags since you are not officially earning that money)