If we display their helical lines, the fit between them is actually comparable to the interaction between an object and an inclined plane. The reason an object placed on an inclined plane does not fall off mainly relates to its material (i.e., friction) and the angle of the inclined plane. There exists a friction angle between the object and the inclined plane; as long as the angle of the plane is less than the friction angle, the object will not slide off. This is the principle behind why a nut, after being tightened, does not loosen automatically. It's because the spiral is similar to an inclined plane, making it easier to apply force; a small force applied along the helical direction can result in a much larger force along the axis of the bolt, allowing for a tight connection of the workpiece.