Search Engine Optimization is a marketing strategy which can often generate a good return for the site. But what must be remembered is that search engines are not actually paid for the traffic they send to a site from a natural (organic) search. Plus, they will regularly change the algorithms that they use for these searches to be carried in an effort to improve the results for people using their search engine. In fact, there is no guarantee that using this system is successful in either the long or short term for any website. Because of this, SEO is often compared to traditional forms of PR (Public Relations), while PPC (Pay per Click) advertising is more closely associated with traditional forms of advertising.
However, even if you do find you have increased traffic to your site because of SEO, if your site is unprepared for this increased traffic, it may in fact be detrimental to your site, as visitors will go away feeling dissatisfied and will not return.
But though SEO can be considered as a marketing tactic which is a law unto itself, it is often considered to part of the bigger picture by many industry experts.
