Search Engine Optimization. There are just TOO many features to list..

Look at the address bar in your browser, notice that the link has search engine friendly keywords (off-topic, vbseo) Also, meta-tags are unique across the entire site (based on content). Again, much too many features to list here!!

Without vBSeo, our friends Google and Yahoo would have a much more difficult time spidering our pages when all the links are based on forum/post-id's, and not relevant keywords.