I have a canon 40d that i shoot with. I hear a lot of people say they have a backup camera and i do understand?

Why. My question is do i get a camera tp backup my 40d OR do i get a another camera and use the 40d as a backup? Idk what kind of camera to get...i like the 50d, 60d, 7d. But i thought about a rebel xti or xsi somethkng like that to back up my 40d. I reallt didnt want to spend a lot of money but i need a good quality backup plan just in case