Well, say you wanted to use one icon for multiple things... then, you could use the icon to identify what KIND of document or thing it was, and then use colors to distinguish them even more...
For example, I could put a book icon on any document related to work. Then, I could color some blue to signify that they're jobs that are done -- red to signify "hot" jobs, or yellow to signify that they're incomplete... or any number of different labeling options...
Once you use them extensively, you'll come to rely on them and they really do help in organization.