The push pin the a smal and inexpensive part that keeps your watch from falling off your wrist. I think using cheap push pins and having your watch falling on the ground can be called penny-wise pound-foolish.
If you wear your watch tight on the wrist, the tips of the push pin can bend after some time. My advise is not to wear the watch too tight and to check the push pins regularly. Change them when you notice declension.
Push pins are sold in pairs of 2, price is per pair.