Why? Well, because by its very nature, the traffic you pay for probably won’t come back unless you keep boosting your post and making it visible. As a result, you need to make sure you have a chance to actually get something from these people on their single visit for your site.
You could try to get some of their basic information — perhaps an email for your mailing list — but the more steps you need to take to get to a sale, the less likely you are to actually make one. That’s why Patel advises to skip boosting the blog altogether and instead focus on paying to send people to your product page.