If we display their helical lines, the fit between them is actually similar to the interaction between an object and an inclined plane. The reason why an object placed on an inclined plane does not fall off primarily relates to the material of the object (i.e., the friction) and the angle of the 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 between the object and the plane, the object will not slide off. This is the principle why nuts do not automatically loosen when tightened. It's because the helix is similar to an inclined plane, which makes it more effort-saving; applying a small force in the helical direction can produce a significant force in the axial direction of the bolt, allowing for a tight connection of the workpiece.