Posts reside entirely within the tooth itself. They extend down into the root, but not out the other end into the bone (at least not on purpose). Their function is to help retain filling material when most of the natural tooth is gone, and you're right, they are only placed after root canals (because they sit down into the space the nerve used to occupy).

When I was younger, they were de rigueur for any tooth that had had a root canal because they were thought to strengthen the tooth. Now we know an overzealous post prep can actually weaken the tooth, and with better ways to bond filling to tooth they are seen less often.

So you see, even dental dinosaurs evolve, albeit slowly sometimes.