Big media publishers like Google and Facebook are first and foremost user-focussed, not advertiser-focussed, so they are looking to prioritize content and ads that are relevant to their audiences and deliver a good experience that help them grow their brand in the medium and long-term. As a consequence, user engagement is being used by Google and Facebook as a metric to assess ads' relevancy; ads are given a quality score that affects the final auction price of our media buying. Hence, the more people that engage with your content, the cheaper it is to boost it.