SEO can substantially increase both your visitors to your site and also improve your sales. It can be the different between your business failing and succeeding. For your business/site to be a sucsess you need to be visible in the search engines.

Also, it is not as simple as getting indexed in to the search engines, when most people search using the search engines they do not go past the first page (the top ten) and very people go past the 3rd page.

For SEO to be affective you need to be in the top ten. It is as simple as that.
Most people who start websites make them to make money, a small percent is for pleasure but most of you wish to make money, either selling a product/service or some kind of adsense, etc. If people are trying to find a site like yours and they do not see you in the search engines then they will think you do not exist.

Imagine you sitting at home doing nothing and a flood of traffic starts coming to your site. You could start advertising, but the let down to advertising is that the traffic stops when the advertising stops, being in the top ten makes sure visitors constantly see and visit your site, as long as you are in the top ten then you are making money.