First of all, SEO stands for Search Engine Optimization.  It is a collection of web development techniques intended to provide a web page with the highest probability of ranking high in search engines.  Now on to your question.

Free advertising is the best form.  No single form of advertising is so targeted as that of search engine listings.  Imagine that you sell rugged cell phones.  Now imagine a customer that wants to buy rugged cell phones for an upcoming expedition in Africa.  He visits his favorite search engine and searches for “rugged cell phones”.  If you’re not on the first page of search results, it’s likely that your phone won’t be in his pocket as he’s paddling down the Nile.  But if he’s sees your website in the top page of search results, you’ve just advertised to a qualified customer for free!  And if you’re website did the best job at creating a positive visit, then you’d better start pulling his order.

Your website was built with a targeted audience in mind, so why not ensure that the audience who arrives is qualified?  That’s the power of SEO.