When you travel for business, you usually keep all your receipts for an expense report later, or to write it off as a business expense on your taxes.
But even someone travelling for pleasure should keep their receipts from all things paid for on the road.
For that matter, every responsible consumer should no matter if they are traveling or not..it's for your own protection. Sometimes business rip you off..this is particularly true of restraunts. Wait staff have been known to alter the gratuity amount. Restraunt owners are also famous for double swiping the card and charging you twice. If you have your receipt, you have your proof when to dispute the charge. Sometimes the product turns out badly, say bad gas in a gas station. Hang on to all that stuff.
I don't get why people buy things and do not keep their receipts.
Dollar, thrifty, avis, alamo..they all state in the rental agreement that you may be required to provide a receipt for gas within x number of miles of the facility. This is standard. As others here have stated, it's to thwart those who cheat the gas gauge.