Well there is always money. You need to know how to add, subtract, and multiply decimals to figure out your correct change. Also, it helps if you can also know how much to tip your waiter. Since almost all cell phones have a calculator or tip calculator, it doesn't really matter as much to know how to use decimals, but it can be helpful. As for me, I usually use my phone's calculator because I do enough math at school daily. Also, decimals on reciepts, house designer draft paper, math questions adults give you, and grocery prices!