Most people will have never visited your website before. How are they going to find out about it? What will be their first indication of its existence?
If the search engine ranking of a page in your website is high enough, then a link to to that page will appear normal search results. These are often referred to as the "organic" search results, on the basis that they are naturally occurring.
If your website has been optimised correctly for search engine access, then the page behind the link will be the one that is most relevant to the keyword that the person was searching on. If the website has not been optimised correctly, then the visit is more likely to be landed on the home page of your website. They will then need to search for what they want. In these circumstances, they are more likely to leave your website and look elsewhere.
Search Results as Advertisements
If the content of the page has been populated correctly, then the link that is shown in the search results will consist of the page title followed by the page URL and then the content of the description Meta tag. These links in the search results are advertising your business and your products or services, in just the same way that a pay-per-click advertisement does in the sponsored links. You have about 30 or so to convince the potential visitor that its worth their while to click through to your website.
If you are using pay-per-click advertisements, then you have even fewer words in which to convince the visitor to click through to your website. You are paying Google for these clicks, so you only want visitors that are likely to be good quality contacts, or are most likely to make an online purchase. To avoid wasting your money on wasted clicks, your advertisement not only has to attract the right type of visitor, it also has to deter the rest.
Horses for Courses
Some businesses are much easier to promote than others. If you are selling a niche market product that has a relatively unique description, such as an expensive ornate gold mirror, then it’s relatively easy to target those who are most likely to make a purchase. If you are offering a service, such as in the finance sector, then it’s much more difficult to differentiate your service from the rest, due to the extent to which many of the keywords and descriptions overlap within this very diverse market sector.