Conventionally, we think linearly about content and keywords; we build a website, and then launch search engine marketing campaigns to drive users to our content.  That approach has its limits. When we think about strategy at Wpromote, we think about  a circular process; since our keyword research reflects both what users are seeking and the way that the search engines (particularly Google) "think" about keywords, we let that help to drive our content strategy.
Additionally, there are many situations where PPC (a component of SEM) makes more sense than SEO. For example, if you are first launching a site and you want immediate visibility, it is a good idea to create a PPC campaign because it takes less time than SEO, but it would be unwise to strictly work with PPC and not even touch search engine optimization.