From the FAQ...

What is reversal film?

Reversal-film like diapositives/slides in still-photography, the film will show the colours as seen through the camera - in contrast to negative-film.

Reversal-film is used when there's only one sample needed. Negative-film is used for spreading/copying the film, because 1) neg film is cheaper than reversal 2) the quality of a neg-neg-copy is better than a rev-rev-copy.

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filmmaking.net
(Incorporating the Internet Filmmaker's FAQ)

Please note the opinions expressed here are those of the author only and do not constitute legal advice. The author cannot accept and liability whatsoever for inaccurate or outdated information contained within.
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Negative film is cheaper than reversal film, but reversal film often appeals to beginning filmmakers because they don't need to make a print, and that saves them money... or does it?

Like filmmaking.net said, use reversal film when you only want one copy... seriously, only one copy. The film that you expose, your permanent record of your shoot, is the same film you put through those meat grinders we call projectors. It's also the same film that you'll edit. With negative film, you make a work print, store the negative, edit the workprint, break the workprint, scratch the workprint, then when you've finally locked your edit, a person in a dustproof room, with lint-free cotton gloves, cuts your negative so you can strike your final print. If this person makes a mistake, he is summarilly executed. Really. You would never dream of putting your negative through the projector. But with reversal film, you just chuck it in there and hope for the best.

One more really important thing to keep in mind in case you're still interested in shooting reversal film: there is no margin of error for exposure mistakes. On negative film, depending on the situation, you can make a mistake of 1-2 stops and the timer will correct it when the print is made. 1-2 stops, maybe more, depending on how much the cameraman feels he can get away with before he is called before the ASC and asked to answer for his infractions. With reversal film, you have 1/8 - 1/4 stop at most, before blacks block up, whites blow out, and your fledgling career goes down in smoke and your house is on the market for half of what you paid. (Yikes... I've got a lot of pent-up fear). (1/4 stop is the sun going behind a thin cloud in the middle of a take). Avoid it if you can. Reversal film is not for the faint of heart.