When NMS first launched, all space craft left visible trails.
In the absence of any effective scanner, it was a handy way to find trading posts on unknown systems. Fly into space, and look for the points on the planet where the trails converged. They would be the points where ships plying between the space station and the trading posts were concentrated.
You could also spot where in the system the pirates were lurking, because their ships left red trails.
HG stopped the trails being visible several years ago, although they’re still there - you can still see them in some rare lighting conditions.