Yesterday I wanted some additional cash and asked for cash-back in the supermarket. I got my 40 euros but was also asked to sign the slip. The shop-owner told me that quite a number of people forget that they've received the cash-back money. Afterwards these people make a fuss in the supermarket and claim that they did not get the money at all. The signature is a simple precaution that makes them aware that they've already put it in their wallets.