The way the computer turns on is the same way that a car turns on.
When you push the power button, it connects the two wires together with a piece of metal (the power button on the computer, the key in the car) so that power can go in one wire, through the metal, and out through the other wire.
There is a device at the end of the second wire that checks to see if it receives power and if it does then it attempts to start the system, whether PC or car.
After that check happens, it doesn't matter where that power goes. It could be routed to many different places to get rid of it. The case wall would be an option. back into the PSU would be one, and through the power cable back into the wall outlet would be another one (combined with #2 option).
None of this really matters. It doesn't even matter which cord you plug into which slot. the cords are stupid and they can be reversed NP. The specs of both cables are the same and they go to the same place, if you reversed them they would just travel backwards through the power button instead of forwards through it. The same result would be achieved.