In the past, many homes were built out of solid wood and similar materials, but most modern day homes have quite a bit of drywall instead. Drywall is much cheaper, and is not prone to many of the problems that wood faces, but when it comes to mounting items on the walls, drywall is pretty much useless on its own. If you try to hang anything with some weight on the wall, you will end up with a hole in the wall, and your item on the floor.
This is why it is necessary to use wall anchors. Wall anchors come in many different shapes and are rated for different weights. Some of them are very simple, while others get in behind the wall with a wide support. Your choice of anchors will be dictated by the weight and size of what you are mounting. Of course, if you are mounting something heavy, if at all possible, you should mount it in a stud, but anchors can be used, as well, if needed.