Unless you have the tens of thousands of dollars necessary to mount a large marketing campaign, or advertise in your local press, you will probably often rely on strangers visiting your website.
These strangers are all potential members or customers of your site, but getting them to your site in the first place is the all-important step.
When looking for information on the internet, most people will use a search engine to find what they are looking for.
Thus, gaining and retaining a high first page ranking in search engines for phrases related to the content of your site is a key step to gaining traffic, and ultimately sales.
Since the vast majority of websearches are conducted using one of the four major engines (Google, Yahoo, MSN & AOL) it is not important to have high rankings in the other smaller search engines, unless they are specific to your industry.
There are several ways you can go about getting a high ranking in the search engines. One method is to use Search Engine Optimization techniques to make your site friendly with the search engine 'spiders' which index the web. Another, more expensive, but effective way of getting a high ranking is to pay for a sponsored listing which will appear on top of, or to the side of ordinary search results.