They are like the cones. The show the entry point, the apex, and the exit point. Basically, if you drive the car through those marker points, you are on a good line (most of the time). The upside is that, unlike the cones, they don't have collisions.
As for authenticity, it's likely not far off in the near future to have AR helmets with information like this being portrayed. Personally, since I have access to real race tracks in the real world, GT is, in itself, and authentic experience of it's own. I accept it for being a digital world race and, as such, I accept the digital advantages.
I sold a track day to a guy once. He lost control, was hit by another participant, and died at the scene. That's a disadvantage to the real world. Since I am happy to accept all the other digital advantages, like not dying, I'm happy to use the markers. They are super helpful, especially when there is a car obscuring your braking reference.