Esty on-site ads vs Etsy offsite ads


What is the possibility that esty ads 15 percentage pay per purchase is how only “offsite” advertising works with Etsy? What is the reason for “Etsy Ads campaign” on sight to be how we pay for seo search placement? It seems like I am comparing Etsy “offsite” and “Etsy Ad campaign” on-site for clarity and for maximization of both….