I want you to do some SERP investigation, meaning perform a search query in Google, see what comes back to you, and then figure out from there what Google believes to be relevant to the keywords searches. What does Google think is the content that will answer this searcher's query? You're trying to figure out intent, the type of content that's required, and whatever missing pieces might be there. If you can find holes where, hey, no one is serving this, but I know that people want the answer to it, you might be able to fill that gap and take over that ranking position. Thanks to Gaetano, @gaetano_nyc, for the great suggestion on this one.
While SEO allows you to optimize your website for a greater chance in appearing higher in search results, and online advertising allows you to send promotional content to the target audience of your choosing, SEM combines both benefits as your content is both optimized for higher search rankings and is placed primarily in front of audiences who are most likely to convert when they click on your ads.