Why should I use tags?

I originally used tags for selfish reasons, to remember my own bookmarks.  Then, I discovered that I could search by my favorite tags and search everyone else's bookmarks under that category (or tag).  This searching by serendipity was an exciting discovery in the world of information acess to me.  It also led me to the third reason to use tags, to show others what you know.  That is to say, I may already know about a great site, but I want others to see it too.  Therefore, the third reason for using tags is to promote a bookmark for others or to market it.  I find myself doing this a lot with the Net2 tag.  If I read an article that I want others to see, I tag it Net2, and I know that people who are subscribed to that tag will see it through their RSS feeds.  I also know that this site will publish it, via their own feed of that tag.

 So, tagging is very exciting to me, as you can see.  My only question is, when will it become a market for advertisers and spammers who will inevitably create scripts that tag stuff automatically.  When this happens, the water will get very polluted.  I fear that day.