The main goal of any web site should be to get attention and get as many people as you can to visit your web site. Whether your website is commercial or not then sooner or later you are going to want to attract visitors. You want the majority of visitors to come from search engines. Certainly, the lowest cost and best targeted traffic will always be brought to you by the search engines.
Search engines make money through presenting searchers with answers they are looking for. The number one goal of every search engine is to attract users and increase loyalty and this can only be done by delivering what the searchers are looking for in a relevant and timely fashion. The more often the searchers return the greater the search engines fortunes. You and your web pages face a similar challenge. Persuade users to come back to your web pages repeatedly.