I know most will say never. Cheater blah blah. But honestly, If you're 30 hours into a game and some rogue single crappy unit sacks a vital resource in the middle of your otherwise unpenitrable empire, or you get beat by one or two turns to a gazilion hammer wonder, or your at war with someone, and your allie signs a peace treaty with the enemy making said enemy their vassel even though they never even so much as fired a shot? I use it when I get robbed of a sure thing just not to ruin the game.