I was never the sound guy (although i do enjoy music (but who doesn't)) so I didn't understand this program as well as LiveType and Final Cut Pro but sound is one of the most important aspects of a successful movie. It creates the mood, action and gives reality to scenes so being able to capture the best out of it is really important.
Soundtrack Pro gives us advantage of having an enormous library of sounds which will be necessary for our movie. Finding or recording sounds would take a lot of our time (which we don't have) and ST Pro cuts that process to simply finding the desired sound by choosing a keyword -> category -> choosing the sound that suits us. It's that simple. It is also possible to edit the sound, change the volume etc.
After all this the sound can be put all together in layers (many sounds playing at once) and exported to the movie this helps to avoid the individual sounds from moving around.