Most websites have their main sources of traffic coming from search engines.  Search engines, the main ones being Google, Yahoo, and MSN, are at the center of online traffic.  Their role cannot be emphasized enough.  In light of the always increasing number of sites on the Web, people all over the world rely on search engines to direct them to the right source of information.

The reason why search engine optimization (SEO) is so popular amongst webmasters is that it is the best way to bring in traffic at low cost.  Not only do you bring in traffic, but you also build your site's goodwill.  That is a very important notion. 

SEO also brings in targeted traffic because webmasters assume that when visitors arrive through search engines, they must have previously entered a search query.  SEO brings in "normal" traffic but also "webmaster" traffic.  What do you think that webmasters do when they want to exchange links?  They search for similar sites.  Again.... using search engines.  Putting emphasis on good SEO allows you to attract targeted visitors, to increase your site's goodwill, and to build visibility with other webmasters as well.  It is almost a self-sustaining process.  If your site is well ranked in search engines, other webmasters will automatically approach you to suggest possible partnerships, which will reduce the amount spent doing "active" promotion.

That being said, it is always recommended that you diversify your online marketing strategy to include other means of attracting traffic, such as pay-per-click (PPC) advertising, pay-per-impression advertising, or pay-per-action advertising.  In addition to entering into online marketing campaigns, you can also utilize traditional marketing campaigns in the "real" world.  Experiment and see what works best.