For about 60 years from the turn of the century to the early 70s these little pieces of iron were most of the protection that was used rock climbing or any other sort of climbing. Eventually Chiounard Equipment kind of turned the tide and intruduced chocks (nuts) and hexs and then Wild Country introduced Friends (cams). Today very few climbers practice the art of piton placement.
There is a good reason so few use them. Pitons are usually solid iron, sometimes titantium. They are pounded into cracks in the rock, breaking the rock and changing the shape of the piton (also known as peg or pin). When they are removed there is a scar in the rock commonly called a "pin scar" where the crack has been slightly widened. As you can probably immagine climber often put protection in the same place and after 50 or 100 or 1000 climbers go part one particular spot the crack which used to be two milimeters thick is now a whopping 15 millimeter thick. Also with modern technology they are largely unnecessary on most free climbing routes.
However, for a select few crazy people they alone serve a purpose. Alpine climbers and aid climbers. Aid climbers often encounter very thin cracks where nothing else will hold them and a piton is really the only option. Alpine climbers often encounter cracks that are iced over and will not hold removable protection. They also deal with rotton rock. In both of these situations pounding in a piton, which is kind of like a big flat nail with an eye hole in it, is the safest option.
Remembering that removing a piton destroys the rock, climbers often leave pitons there so that future parties can use them. In fact this is such a common practice that most trade routes have fixed pitons or fixed pins at all the necessary places. So there is really no need to even have pitons unless you are trying a first ascent or obscure route or some crazy twist to a route like doing an unusual route in winter or something. So taking the responsibility to place pitons is like a right of passage. It is something that is earned. Anyone can buy pitons on the internet and go out and use them, but the sport of climbing has advanced so far that these pieces of metal are now used by very few people. It is an interesting and unspoken twist to the world of climbing. You should not be seen with pitons unless you are doing something hard enough to demand their use. It is like a cowboy hat among real cowboys. If you are a pretender do not wear one. The same with certain color shirts in certain cities. If you are not a part of the gang do not wear it.
So that is my spiel on pitons. I only own four. Just in case I try something ridiculous, well more ridiculous than usual, and have to repel a bunch this winter. I'm not planning on doing any routes obscure enough and hard enough to have to use them, but we shall see...