Being a schmuck and not having to pay tolls. It's stealing from your fellow taxpayers.

Actually at the time I did not drive on any toll roads.

Honestly I was mostly just lazy, I can't think of any real advantage. I guess if I accidentally went late through an intersection and would have been subject to a red light ticket, I would have been saved on that. However I think this has never happened to me, so this is not a real conscious reason for me.

Surely they know from the dealer whom that plate number was assigned to. In fact: when they send the metal one does it have the same number?

In 2017, California did not have temporary plates. The dealer plate just said "Honda". To track the car, the dealer tapes a very small piece of paper to the inside of the window, so if you are pulled over an officer standing right next to your car could read that to verify your car is registered etc. But they are not going to do that unless they have some other reason to pull you over.