Tags are really important for a couple of reasons:
1. they tell the search engines what your page is about
2. they create internal links to your pages
This is a very good hub on using tags effectively that every hubber should read; http://hubpages.com/community/Hubpages-Traffic-Use...
I don't know the ins and outs of how hubpages internal search works, but I think several factors are used such as words in text, words in title, words in summary etc. and not just tags.