The Smith Machine is a weight training machine. It is unique in that it is a free weight barbell, but it can only move up and down. This gives it an extra dimension of safety for the beginner bodybuilder.
The additional safety is achieved through several lockout positions. Adjustable safeties allow it to be secured at any point. The benefit being that it is safer for those who train without a spotter. If the weight becomes to great, you can twist your wrist and lock the barbell in place. Of course for maximum safety it is recommended to always train with a spotter.
The barbell does not need to be stabilized as the Smith Machine does not allow it to fall forwards, backwards or sideways. This enables you to life more weight, but don't be lulled into a false sense and don't lift more weight than your body is ready for.
Using the Smith Machine is not the same as using free weights. You can not do exercises that require an arc and you do not get the additional benefit that comes from having to stabilize the weight as you lift it. Stabilizing the weight is an important component of bodybuilding. Using the required muscles is required to develop them as well. This is the challenge with it comes to doing squats and bench presses on the Smith Machine. In that regard you would not want to use this as your primary workout machine, but it comes in handy when you are alone.
So the Smith Machine has it purpose, but it is definitely not a replacement for free weights. Beginner bodybuilders need to build muscle mass not just on the primary muscles, but on the stabilizer muscles as well. Free weights better imitate the range of motion that you would actually be performing in real life activities, but when you are alone or can’t find a spotter, the Smith Machine offers a safer alternative.