Why should I add 'tags' to my blog post?

Think of tags as the colorful little page markers you might use to flick back to your favorite pages in a book. The tags don’t describe the book as a whole, instead they describe individual sections of the book.
The tags you add to your post are used in the tag cloud. A tag cloud or word cloud (or weighted list in visual design) is a visual depiction of user-generated tags, or simply the word content of the site. The tag cloud is easy to scan. The cloud works because it fits a lot of information into a small space, and is easy to scan over.