Why are two door signal wires being necessary? Why is single wire not enough?

In that application one wire is for the door ajar signal that shows on the dash. The other one goes to the BCM to tell it to turn on the interior lights and can work with the locking system and alarm to tell those if the door is closed. If you look at the wiring you can see that the green wires all connect from both front doors and rear doors, so that if you open any door, it turns on the dash light. They don't use that linked signal because it wouldn't tell the BCM which door is open. So, you get 2 signal wires, but each serves a different purpose.

Actually, there are three inputs from the door switches to the BCM. Left front to pin 6, Right front to pin 7 and then the door ajar input that goes to pin 17. Each one is a different piece of information to the BCM, and it uses those inputs to control multiple other pieces in the car, not just to turn on the interior lights but to also activate other items. Say you open only the right rear door. That only triggers on pin 17 so the BCM only turns on the interior lights.
Now you open the driver's door, the BCM sees the signal on pin 17 plus the signal on pin 7 and now it turns on the interior lights and activates items that the driver might need. It also can turn off other options, for instance a time delay on the interior power, you turn off the key and don't open the door and the PCM may allow the radio to work for 5 minutes. It all depends on the options and what they designed it to do. The other part of that is that the second wire may also carry extra data, for instance telling the BCM if the door is locked or if the outside door handle is being used. Look through the rest of the info and look up the BCM inputs and what they do, and you might find out more.