A switch will recognize a PoE device and push the full 15.4w by default. Then the device has the abi6to communicate back to the switch to specify how much power is actually needed.
I think the efficiency greatly depends on your switch efficiency and how many devices you are going to power vs the efficiency of your PoE injector or wall wart.
There is also a convenience factor of not needing to plug something in locally.
Also, security, someone can’t just unplug the power supply for your camera per se.