If it's a carbon steerer you don't want the stem clamp applying force to the top edge of the steerer tube, the theory being the force can crush and crack it.
Placing a spacer above the stem means all clamping force coming from the stem goes into the walls of the steerer more evenly, reducing the probability of anything nasty happening.
If it's a carbon steerer you don't want the stem clamp applying force to the top edge of the steerer tube, the theory being the force can crush and crack it.
Placing a spacer above the stem means all clamping force coming from the stem goes into the walls of the steerer more evenly, reducing the probability of anything nasty happening.