Okay, when we talk about the "why" of tagging - we use two words - findability and discovery. I love the way Marnie Webb phrased it in this comment.
"Discovery allows me to play. Wonder through tag streams and find related items in a way that helps me get at various topics in a much different way than search."
On the other side, I have a colleagues who hate, no HATE, this part of the tagging experience and think it is a big fat waste of time. One colleague who I respect said this to me: "Although following other people's tags is interesting, so is wandering around the library looking through books in certain sections, but it is a huge time vacumn, considering the actual usefullness and applicability of what I find is usually nil."
This is what think - I think it has to do with learning styles and myer briggs personality types. I haven't come across specific research, but Nancy White mentioned this in her presentation about the Seven Competencies of Online Interaction that "Interacting online involves thinking/moving laterally not
hierarchically. Global thinkers, those who absorb large amounts of information randomly and then see patterns, do better than sequential thinkers who tend to take linear steps that follow in a logical order." So, how can we make tagging easier for sequential thinkers?