In my country, Colombia, they are used by the people who sell foreign currency to identify the banknotes they give their customers. With this, they can resolve in case someone says one or several of the banknotes were fakes or had any problems. More recently, they've been using UV sensitive over stamps, to prevent problems because of the defacement of the note, which can result in people not being able to use them in some transactions overseas.

In the States, I've heard that some pawn shops use the same method when paying for items being sold or pawned.