Search engine optimization is a marketing channel like any other. The "ad" in the case of SEO is your own quality content and optimized pages.
But how can you measure the value of doing more SEO work compared to the more clear decision that ad campaigns offer? If an ad campaign costs $2,500 and delivers $10,000 in sales, you have a return on investment of 300%. But how do you measure the value of a blog post when you have traffic coming in from multiple sources? How do you measure the value of the post when traffic comes in directly, from search engines, from referral sites, and from you own social media mentions of the content.