Because you not just optimize the all people in the basic “view content” event, you separate the irrelevant people by additional data and optimize only the relevant people, then you will get much better results.
Because you’re targeting only relevant people, so they are much more likely to engage with your facebook ad, and facebook likes engagment.
Because you separated the irrelevant people before so now you promoting only to people that you know they more interested in your offer, then you probably will get higher CTR on your ad which means these people will be more engaging with your facebook ad, because you relevant to them. (so you pay less for clicks)
Think about that, if you optimize the “view content” that is the first step and most important in the funnel in the wrong way, then you throw away your money and the ROI will be minus. but if you optimize it like a pro then all the funnel will be smooth and you will get positive ROI.