When you can not get far enough away to fit everything in the viewfinder
Add surroundings to a shot of an event which has now passed
Make large images suitable for the "Ken Burns" effect in iPhoto or FotoMagico
DoubleTake is the "nano" of stitching software. Drag & drop your images onto or into DoubleTake, arrange them, and check the overlaps. When you are done - save and perhaps copy and paste the result to iPhoto.
DoubleTake is simple enough to not require a manual, but a few annotated screen shots highlight a few main points.