Light sockets with a built-in switch were once very common--and they are definitely still around. It is best to mount these light sockets--or any light socket--high in the room. This not only gives the best illumination but protects the bulbs. Unfortunately this makes it hard to reach the sockets to turn the lights on and off; thus, the pull chain switch was born. A chain (or string) hangs down from the light socket to a height that is more convenient.
The first patent for a pull chain switch was awarded to Harvey Hubbell of Bridgeport, Connecticut in 1896. These soon went out of favour because they quickly broke; part of the problem was that people pulled too hard. Eventually, people learnt how to pull them correctly, and the pull chain switch became more common.
The function of a pull chain switch is to solve a single problem. Some devices (notably lights and fans) function best when they are mounted on the celling. It is convenient to have the switch be part of the device, but it is inconvenient for humans to operate switches that are mounted on the celling. Pull chain switches solve this problem by allowing people to pull a chain that operates the switch.
There are basically two types of pull chain switches: toggle switches and multi-position switches. Toggle switches change state (off or on) with each pull of the chain. Light switches are usually toggle switches. Multi-position switches cycle through each of several states with each pull of the switch. Fans often have multi-position switches for off, low speed, medium speed and high speed.
Pull chains do require a small amount of easily learnt skill. Besides the problem of pulling too hard and breaking the chain, there can be a problem with the release. Occasionally, when the chain is pulled and released quickly, the action of the switch can flip the chain up so it becomes stuck out of reach. The solution is to hold onto the chain until after the switch has changed state.
Pull chains can be difficult for children (or people in wheelchairs) to operate if the chains are on ceiling-mounted devices. If this is a problem you can extend the length of the chain, although this means the chain may hang down into the path of anyone passing under the device.