Number 10 – SEO is not Advertising
Number 9 – SEO has more credibility
Number 8 – SEO has the potential for viral spreading
Number 7 - SEO forces you to organize your site
Number 6 – SEO makes the user experience better
Number 5 – SEO introduces you to Google
Number 4 – SEO establishes authority
Number 3 – SEO builds links to your website
Number 2 – SEO will boost traffic to your site

AND the Number 1 reason...