Why Slide Film?
Slide, or transparency, film has many advantages over print, or reversal, film. To start with, the chemical properties of slide film provide a close-grain image quality, unequaled by print film. Most professionals use slide film, mainly for this reason alone. Close-grain quality becomes increasingly more important the more a photograph is enlarged. Keeping and storing slides instead of prints and negatives also simplifies the task of sorting and reprinting the images from large catalogues. (When a print is needed, it can be made from the slide at any time.) Also, when viewing slides the photographer sees the original image in its purest form. Because of print film's wider exposure latitude, the exposure you intended may not be accurately rendered in the final image. If developing is not done by the photographer, the original image may also be further altered at the lab during printing, usually without the photographer's consent or knowledge.
Slide film records exactly what you exposed, regardless of who develops the film. While to some this may be a drawback (since this allows no room for error), to others it is the best medium for recording a meticulously calculated image. In the art and commercial fields, this kind of control is crucial. This is also why slides are ideal for testing photographic equipment to be sure of it's accuracy. And for educational purposes, since slide film doesn't lie, it's the only way to know that you've exposed your image the way you intended. You can't master the art of photography until you've learned proper exposure techniques.
It is for these reasons that most photographic competitions use slides. Slides also allow the ability for several judges to view the image simultaneously, under the same conditions, and in the same format. This way only the photographic image itself is judged; not the mount, border, or print size. And since no printing or mounting is required, this saves the photographer much time and expense.