In the Facebook auction, what is the purpose of ad rank?

In the Facebook ad auction, ad rank determines which ads are shown to users and in what order. It's a score calculated by Facebook based on several factors, including the advertiser's bid, the ad's estimated action rates (likelihood of engagement), and the ad's quality. The purpose of ad rank is to prioritize ads that are both valuable to the advertiser (high bid, good action rates) and relevant and engaging for the user (high quality). Ads with higher ad ranks are more likely to win the auction and be displayed to the target audience. For instance, an ad with a slightly lower bid but higher engagement rates and quality can outrank an ad with a higher bid but poor user experience.