Tagging allows you to express your opinion about content and make your judgments, opinions, and identity part of the system.
In social tagging systems, some tags serve a dual purpose. Take the tag “funny,” for instance. When you use it, you’ve created a way to re-find something you thought was funny. But you’re also telling other users of the system about what you find humorous, and you’re communicating something about who you are.
At the music-sharing site Last.fm, many people use the tag “seen live” to identify the artists they’ve seen and, presumably, to tell other users who they’ve seen as well. It’s like the online version of wearing the T-shirt you bought at the gig.