This is a very effective exercise for your lower chest area. The Smith Machine provides for a very smooth and fluid motion that allows you to execute this movement with ease.
While the Smith Machine can be used to build muscle mass, we suggest using the barbell (free weight) if your goal is building muscle. There is nothing more effective than free weights for packing on slabs of muscle.
Thus, the Smith Machine is very useful when you want to create muscle separation and work on definition and toning. It's also good if you simply want to change things up a bit to break the monotony of any given chest workout routine.
Finally, with the Smith Machine you can isolate the movement to the point where you hit the isolated lower chest area which will result in giving you a nice bottom outline to your chest. Something that looks very impressive and superhero-like.