I've read that the Prius 12v battery is only used to boot up the computers. Does it do anything else? What runs the lights, AC, accessories, etc. when the ICE/MG1/MG2 are off? If all it does is boot the computers, where does the power come from to start the drivetrain elements? Why can't the Prius "jump start" itself by starting and then charging the 12v using the main drivetrain?
If you are unwise enough to run the lights, AC, or Accessories with the car not in READY Mode, the 12 volt battery does, briefly. Besides booting the computers, it powers two relays that allow the HV battery to connect to the inverter and on to MG1 and MG2. To be safe for first responders in an accident, if the car is not READY, all HV wiring is dead. https://techinfo.toyota.com/techInfoPortal/staticcontent/en/techinfo/html/prelogin/docs/2ndprius.pdf
There's a DC/DC converter that reduces the HV battery's 200+ V to the necessary 12V to power all the electrical accessories. Once in ready, the 12V battery just serves to steady out the 12V power a little bit, and provide power (or a place to sink power) when there is a sudden load change. As Jimbo said, it powers two relays that connect the HV battery (you can hear them click in the hatch area, if you listen closely, when the car turns on and off). Once that's connected, it is used for power to start the engine when needed. And yes, safety is why you can't "jump" it from itself; it disconnects the battery for safety while off, so it needs a separate power source to reconnect the battery (after a quick check of the computer and electrical systems indicates that doing so is safe).
The 12V battery provides "regulation" or "ballast" to the 12V system, which powers the computers, lights, radio, etc. It is charged by a charging circuit in the inverter when you are in "ready". As stated above, when starting, it also allows closing of the relay that connects the 200V battery to the HSD system. It isn't used to start the engine directly. The engine is started using the 200V battery, the inverter, and the electric motor/generator #1 (a 20 HP electric motor). Because of this there is no "bendix" gear, no ring gear on the flywheel, etc. A MUCH better system. Good thing, because the HSD often shuts down the engine and restarts it as you drive. It's one of the ways the Prius uses to get such good fuel economy.
