Tagging can be useful when you are dealing with many examples of similar pages or references. This reminds me of an electronic thesaurus. Instead of see also, it will pull up all items with similar tags. Very much more user friendly than the Library of Congress Subject headings. Uses "human" language/slang.
Delicious is fabulous. I'm not sure if I want others knowing exactly what I'm bookmarking and tagging, but it is very handy to be able to access your bookmarks from any computer. I will definitely be adding my most used to delicious.