Here is a summary of Sigma's functions
1. Creating noise-free high S/N darks. Most people create a master dark from the median of a set of darks because using the average of the darks will transfer all the noise from cosmic ray hits into the dark-subtracted light image. If you use Sigma you can create a master dark with the overall S/N of an average with the noise rejection of a median combine.
2. Creating superior flats. The normalization routine in Sigma is sophisticated and works very well. You will be able to create less noisy flats that will work better on your images.
3. Create superior combined light frames. You can also quickly compare combines of the images that represent the Min, Max, Median, Average, and Mean of Medians.
4. Soon you will be able to reject hot pixels from the images with a single button click.
5. Soon it will be able to fix blooming stars.