This may seem absurd, but let's look at things from Google's perspective for a moment. Google wants to provide the most relevant, non spammy and informative content possible to anyone that uses their search engine. They achieve this by providing search results that are relevant to the keyword phrase a person is searching for. The first sites Google will be displaying are those that they believe are "authority sites" for the keyword that the person entered.
A long sales copy landing page about an Acne Treatment eBook isn't going to hold the same authority for an organic listing as an educational site that explains the various medical conditions associated with acne. At least, not naturally. That's because naturally the educational site will receive more backlinks from other websites, from people who think that the educational site has something to offer. It's unlikely that people will be linking to a commercial sales letter (not naturally anyway). That's where we change things as Internet Marketers. We manually create these backlinks ourselves, in order to pump up that Acne Treatment eBook landing page to compete with the educational site.
One way we get these backlinks from sites, is by submitting articles to multiple article directories and placing a link within the article that leads back to our landing page. Now remember when I said that we need to include relevant and helpful information within an article as opposed to sales copy? That's because Google will be determining our Keyword Authority based upon the source content of all our backlinks.
If Google sees the source of all our backlinks as being articles that advertise the same product through repetitive salescopy, we risk being flagged by Google as being spammy, which is something we don't want. Furthermore, many article directories will reject articles that are advertising a product, or that contain an affiliate link. Remember what our current goal is: it isn't to drive a sale (not at this stage), it's to create a relevant backlink. It's important you grasp this concept.