What is the purpose of a star nut?

Question

What is the purpose of the star nut and compression bolt combination in the steerer tube of the fork?

Answer 1

Older threaded headsets had two locking nuts that put proper compression on the headset bearings so the forks weren't loose.

On a threadless steerer tube, there are no nuts to do that job since the steerer comes all the way through the head tube of the frame and gets clamped onto by the stem.  So the star nut goes inside the steerer tube, and the headset cap screws into it to properly load the headset bearings BEFORE the stem is tightened onto the fork's steerer tube.

Once the stem is tight, the top cap can be removed without consequence,but if the stem is EVER loosened, adjusted or removed that cap needs to be there for bearing preload again, so just leave it be.