A Wiki is a great collaborative tool. It enables people to work together on a document anytime/anyplace - because the document is on the Web. And, they are spared that nasty morass of "track changes" of multiple colors and side notes that you have seen in Word documents that area emailed from group member to group member in endless cycles.
With a Wiki, someone initiates the document and shares it with the rest of the group. Anyone can make changes at any time. All prior versions of the document are stored in the history, so one can revert to a prior verision at any time.
Imagine a student group project. Have you ever encountered a student who reports "I did all of the work - Ray did none!" One of the great aspects of a Wiki is that you can see just what contributions and edits came each member of the group!