In a threadless fork the spacers and the stem clamping around the steering tube, allow for headset bearing pre-load when loose
and holding it like that when tight.. spacers fill in the gap. under the stem & thansfer the adjustmrnt force thru the stem

a stem raiser for threadless is when the last guy cut the fork steerer too short...

a Quill to threadless adapter is a plug to grip the stem around that goes in the fork/.