Slides are great to view as they have a much greater brightness value than prints, because you are viewing them by transmitted light (as are the pictures on your computer screen) as opposed to reflected light for a print. I think all the ways they can be used have already been mentioned. However, may I add that if you are doing alternative processes that require contact printing with a negative, then starting with a positive transparency means you can go straight to neg, instead of neg - pos - neg - pos.