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the stem is keeping everything in place.
Until your frankencup tries to lever itself out of the head tube and pulls the steerer through the stem. In practice, all AHead stems try to walk up the steerer on load reversal, and leaving the top cap in place seems to help to prevent this, so your headset preload doesn't go out of adjustment as quickly. I add more bolt tension to the preload adjuster after clamping the stem to the steerer, to make things even more secure.
gbj_tester
When you've (properly) fitted everything the top cap is unnecessary - the stem is keeping everything in place.