It's what distinguishes a contract from a promise. If I say, "I'm going to give you some apples in six months, after the harvest" and then there's a blight and

It's what distinguishes a contract from a promise. If I say, "I'm going to give you some apples in six months, after the harvest" and then there's a blight and I don't actually end up with any apples, society (at least in America) decided that I should be able to just say, "Oops, sorry, I'm not going to be able to give you those apples after all" and be done with it. On the other hand, if I say, "I am going to sell you some apples in six months, in return for $100", American society collectively decided that I'm on the hook to get you those apples, regardless of whatever difficulties should ensue.