You've invested the time and money in creating or buying your food. Why wouldn't you save it for another time?
While it's starting to become universally accepted, we need to make saving leftovers cool. It's certainly practical--today's dinner remains can be tomorrow's lunch or dinner. But it's also the green thing to do. By storing leftovers, you're keeping food out of the landfill and making sure that the energy used to produce it doesn't go for naught.
Leftovers also provide a chance to be creative in repurposing them into another dish. You can even make a game-show-like challenge out of it, by seeing how delicious a meal you can create from your fridge contents. Including an Iron Chef-style "secret ingredient" is optional.
Finally, whether they're from a home-cooked or restaurant meal, don't forget to eat the leftovers you save! It doesn't do much good to store them in your fridge only to throw them out a week or two later. For more tips visit my website, American Wasteland.