Anyone who writes content that is supposed to get a high rank on Google knows there is on-page and off-page optimization. Your on-page content has to answer the questions that led visitors to your site. Off-page optimization is backlinks and guest posts. I have heard of companies spending tens of thousands of dollars per month on backlinks. I would say they are absolutely necessary if you are creating content in an industry where it's very difficult to get organic backlinks. They can also backfire horribly and make Google punish your site and restrict it from appearing in search results if their algorithms conclude you paid for those links.