What is a two dollar bill used for?

The same uses other money has: buying things, paying off debt, and others that can be attributed to this. It simply has a different amount than other bills, but is not nearly as common as other denominations, making up only about 1% of all bills in circulation. In spite of that modern $2 bills aren't rare; hundreds of millions have been printed. While that's a large number it's still far less than the tens of billions of other bills (mostly $1.00) in circulation.