This prevents feeding more fuel than the carb can use which in turn leads to better emissions and fuel economy but personal opinions again deleted. The slide is controlled by positive and negative pressure differences.
Air moving past the hole creates a negative pressure above the slide.
This is done in proportion to the pressure differences between the upper and lower chambers of the diaphragm and automatically keeps the air at a constant velocity. The throttle plate controls the amount of air and the slide controls the velocity of the air.