A decent rear-projection setup is better than the alternative, which is green-screen. Green screen is expensive, labor intensive, and takes a lot of time to turn around. Most of the time they'll do rear-projection instead.
To do rear projection, then set up 3 screens in a U shape around the car. Two on the side and one in the back. They'll either have filmed the "backplates" some time before with a camera truck, or, rent a ready-made set of backplates from a stock footage company. Backplates are tricky, because of a speed illusion. Basically, if the rear shot is filmed going 30mph, you want the side shots filmed at 15 or 20mph. If the speeds match, the scene won't work, for some reason it just looks weird... like the sides are a blur and the rear is in focus. So you need to go about half-speed on the side shots.
Anyway, the thing is, you can't really film a long backplate. How many roads are there where you can drive a long time at 30mph without stopping for a stop sign or red light? And where the beginning of the trip would look the same as the end? Because what happens is, they'll shoot a scene in a bunch of takes, and the actors are concentrating on acting and can't see the background behind them. There's no way to get them to deliver a line at the same spot in the stock footage... like "say it when you pass that tree" or "when you pass that restaurant sign" or whatever on every single take. So they just let the background plates roll over and over, and they might spend 30 minutes or an hour or even two shooting takes in the car. Eventually, it's gonna loop, and it's just dumb luck if people notice or not.
If they really don't want the loop to happen, they have to shoot it with greenscreen and put the background in afterward to make sure it doesn't repeat. But greenscreen can be even more jarring to see than a loop, so most of the time they just gamble on it.