Why do I need Search Engine Optimization (SEO)?

If you've every had a website or dealt with anyone that knows anything about marketing, you have undoubtedly heard the term "SEO".  It's likely in fact that you've had your fill of "experts" wanting to help you with it, audit your site, tell you why your results are lacking and a myriad of other tag lines attached to these three letters. Well, so what's the big deal about it and why is it important to me?