Hm. This you?
You said they don't need to exist. Just, they don't need to exist. Even if we accept that's not what you mean and amend that to "they don't need to exist for racing", that's still wrong. They're part of the racing too.
If you can't identify the track you're on without them, that seems relevant for starters. It's important in racing to know where you're going and which corners are coming next, and landmarks help with that. In a long race or a tight battle it's not unheard of to lose sequence of where you are, especially at night. Likewise, objects near the track but not on it can be used for things like braking markers, turn in points, apexes and sight markers, which are all quite relevant to racing.
When trees and other trackside objects change sightlines and block visibility around corners, that changes the racing. It matters whether or not you can see the big crash around the next corner before you're in it.
When they're the difference between seeing clearly and getting blinded by low lying sun, that changes the racing.
A track lined by trees and buildings doesn't see the same wind speeds as one that's open, and that changes the racing.
There's a bunch of other stuff as well like track temperatures and drainage that probably don't even come into a game like Gran Turismo, but they apply in real life and they can apply to other more complexly simulated games.
The bottom line is that for a ton of reasons, a plain ribbon like the one in the video
@Samus posted simply does not race the same as one with a properly detailed surrounding environment even if the road itself is exactly the same. Changing the external environment of a track changes how it plays and races, and that's what you'd learn if you ever bothered to try and race on a plain ribbon with no surrounding environment.
And so given that all this stuff needs to exist for a track to race the way it does, and given that actually a good driver spends some non-trivial amount of time using these environmental cues to help them drive well it's relevant that they look appropriate. It's why it's actually fairly cool that Polyphony bother to try and locate vegetation in roughly the correct spots around their tracks, and to model trees and such that are pretty close to what appears in real life.
And if the player is going to be looking and and using this environment in this way, it's also meaningful if the environment both fits in with the high quality of the rest of the track and the cars so as not to be jarring or distracting, as well as just being pleasant to look at in general. Yeah, you could put up a bunch of cardboard cutouts and get largely the same physical effect, but the sense of realism that the game is going for would be substantially diminished. Gran Turismo prides itself on realism and providing an experience that
feels realistic, and so that alone is more important to the game than it might seem.
Still, I know there's bad players out there that never look up from the track just in front of them and would never bother looking beyond the fenceline. They exist in real life too. They will not understand this, but good racers use all the tools available to them and that includes stuff beyond the driveable surface.
