When you tighten the stem, the fork is locked in the right position.
Why can't these parts be removed to save weight for free???
Just glue the cap on?
It depends on a couple of things, one being the strength of the steer tube and how much it might "give". The clamping force of the stem works with the expanding force of the "expander", sandwiching the steertube in between. I wouldn't even think of not using a good expander in a carbon steertube. Try this, tighten the expander quite a bit, while the stem is not on the steertube. Then try to put the stem on... can you even get it over the expanded area of the tube? Sometimes you can't, indicating how easy it is to expand the steer tube or conversely, crush it. Well, same thing in reverse... if there's no expander in there, the stem can come loose and work it's way up so much easier since there is nothing on the inside pushing it against the stem. The top cap is just there to provide initial preload until the stem gets tightened down. Once the stem and expander are both tight, it does nothing. It only provides a preload prior to torqueing the stem in the right spot.
So it's very dependent on the design of the steertube... was it intended to used without an expander, like an alloy steer tube, where just a startnut is used to provide an anchor for the top cap bolt and provide preload, or is it a light carbon steertube where the expander serves the dual purpose of preload AND reinforcement. Come to your own conclusion on your specific bike, but don't lightly dismiss what the original manufacturer has supplied.