Submitting your site to main search engines is not enough untill you dont try to optimize it . So search engine optimization is as necessary as submitting your site to google and yahoo !
Search engine optimization is the art and science of making web pages appear attractive to the search engines. The better optimized a website is, the higher the ranking it will receive from a search engines web crawlers, the higher its ranking the more traffic your website will have, the more traffic your website has the more profit your website will generate. The key is good internet search engine optimization.