Organic Search Engine traffic is where people reach at your website since they typed in a phrase or a keyword that is relevant to your website, and because your site had proper optimization, your site shows up in the first few positions of the search results and gets the click-thru from the searcher.

The reason why one should concentrate more on Organic Search Engine Rankings are:
It is completely Free! (You need not have to pay for traffic)
100% Targeted! (What could be more targeted than a person who actually SEARCHED for what you're offering?)
Completely auto-pilot! (You don't have to send any emails or place any ads. Your visitors will come throughout the day and night regardless of where you are or what you're doing)