It loads scenery at full quality even where you’re not looking, i.e. behind you and to the sides.
It reduces pop-in and stutter/FPS drops when moving the camera around, at the expense of higher CPU usage (possibly reducing average framerate a bit), and higher VRAM and RAM usage (which is good as long as you have it to spare).
If you have issues with low framerates while flying, a lower setting may help.
If you have stutter or pop-in when moving the camera, a higher setting will help.