I get the two mixed up, I know one is organic results whereas the other is more marketing based, but to me the reason that you want to optimize your site for organic results is to market it better. Someone shed some light?
SEo is the process of optimizing your site through onpage and off page methods so that your site ranks well in the Organic Search Results for its specific keywords. SEM is paying to have your ad/site shown.....thats all.
Regardless of method if you are an online business your goal is to find customers and convert those customers into income. Organic Search has a cost, Paid Advertising has a cost. You do what makes the largest return on your investment. Usually that includes taking advantage of both.