In short, friction. Specifically, its function within screw mechanisms designed to hold things in place.
For me, the most apparent example of this is on dumbbells where one has to put on weight plates and then screw a ring in place to hold the plates. If the screw were "ideal," it would have no friction, which would be great except for the fact that it would no longer be a functional fastener.
By screwing the ring tight against the weight plates, a force is applied pushing the ring "out." In the absence of friction, this would push the ring out and be converted to a torque by the threads, spinning the ring away from the plates. Luckily, the friction between the plates and the ring mean the ring can't spin, and the friction between the threads on the ring hold it laterally in place.
More generally, screws in their traditional sense would be utterly useless without strong frictional forces holding them in place. They would lose their holding power and simply unscrew themselves as soon as a force was introduced.