Main thing of SEO and SEM..
 

SEO is used to optimize the search engine to increase the traffic to your website through On-page and Off-page optimization. Whereas SEM is used only to advertise your website in the internet through various techniques the only purpose of SEM is increasing the traffic through marketing in the internet....

Yes SEM is best way to increase traffic but beside that when site will have traffic then there will be more chance to get more leads as well.

You can use seo for generate the back links and you can use smo for generate the traffic.

Thnaks for sharing such a valuable information with us.