The answer to this question should be easy: they are both thought to present your product to others. It does not matter that the products are real life things, services or the content of your website itself, they all have to be served to other people (your clients, your readers, your viewers) in a manner they notice, remember, love.
Always learn to find out more about the rules of SEO. They change in time, but their base are always the logic (reason), the common sense and... the previous experience. The searching engines have logic designed by their creators, they try to present some common sense information to the people using them and they change according to the experience gathered from the interactions with the webdesigners, the content writers, SEO experts and, especially, the users.
The online marketing does not work without the content of your website, so does the SEO. The marketing is targeted to specific areas, populations (cultures) and age, so does the SEO (even if its targets are in some ways different).
Finally, but not the last: they both have to have some targets as number of clients, of visitors or viewers.
