Because some generating units require electricity themselves before they can be brought online. A good example is a nuclear station - you need electricity to run the cooling pumps and withdraw (carefully!) the control rods, before steam generation starts and the turbines start spinning.

While nuclear plants do have diesel generators for this, it'd still be a good idea to have one or maybe two sections of the country which could bootstrap the rest of the North American grid(s) should there be a continental blackout.