The majority of companies advertise and Search Engine Optimization (SEO) is essentially advertising in a different form. With Pay Per Click you literally pay every time anyone selects your advert and once you have reached your budget limit your advert disappears from Google. SEO is about your website and promoting it, organizing the content and structure into a user and search engine friendly manner with the express desire of seeing it move up the search engine results ranking with the long term goal of being found first on Google for your chosen search terms.
With the question of ''why do SEO?'' there will always need to be a business decision between the short term gains of PPC advertising and the long term benefits of being found at the top of a search term. This must also take into account the time and cost of reaching the top of a search which varies greatly depending upon how popular the search phrase is
Although there are never any guarantees to be made with organic SEO there is the simple fact that a successful SEO campaign can have the website showing up in the search engines results 24/7/365. Where as paid for advertising will only last as long as the budget lasts to continue paying.