The expander is intended to be used when you are gaining and compressing your incoming signal as this in turn brings up the noise floor.
By using the expander you can supress the noise that could appear when boosting your signal. You would notice this noise especially in silent passages, where no actual sound is present.
When the signal goes below the Expander threshold setting, the signal level is reduced, resulting in less noise. Using the expander should make your compressed signals more clean.
A voiceover track is a good example of this, as people normally apply compression quite heavily there.