As mentioned, it improves visibility. Originally it was designed for computers (not just laptops) with crappy screens/refresh rates, since fast moving mouse pointers can be hard to see when the screen is dark/flickering/slow at refreshing (often all related). The other aspect is that some people have bad visual acuity and find it harder to see/track fast moving objects like a mouse pointer. Trailing the mouse makes it much easier to see, even on crappy screens, even if your eyes aren't the sharpest.