What is the main purpose of SEO?

The purpose of SEO is to reduce the time a person spends searching for a relevant result.

The purpose of SEO marketing is to spread the word, thats it. However which word or which message is entirely up to you

This is accomplished through refinement and streamlining of various aspects of the target site and page ranging from the programming platform, the hosting environment, site architecture, tweaks to images content and tags, link popularity and most importantly the chronological order of events that transpire to cultivate a site into a top 10 candidate .