48V isn't what enables Ethernet, it does however make it more useful. 48V enables high power things to be scattered around the car - voltage loss from the front to the back of a car at 12V is an issue, and cars spend a lot of money on large wires to work around that.

High power things scattered around the car often can take advantage of Ethernet. things that cannot get a lot of power can get by on lower bandwidth data buses.