I can't give you exact details on why it works, but I do know that the ignitor is how the ECU communicates with the distributor. It's a type of relay.

And, to answer your other question, if the ignitor is bad, you get zero spark. So don't worry about whether it's bad. If your car starts, the ignitor is fine. Also, if it does go bad (which is not totally impossible), it's pretty easy to replace, it's held in by two little bitty hex-head screws. Just make sure to write down which wire goes where, cuz the ignitor has 4 different colored wires comin off of it.