Before looking at the Smith machine home gym lets firstly look at what is a smith machine in the home gym context.
Essentially a smith machine is made up of a barbell which is fixed within steel rails and which only moves vertically. It was invented by Jack La Lanne who fixed one up in his gym in the 1950s and bodybuilder Rudy Smith subsequently improved on the original design.
The major benefit of the smith machine is that you do not need a spotter when working out because there is a series of slots on which the barbell can be hooked and secured. You only need to twist your wrist to lock the barbell in place thus increasing safety enormously for weight lifters.