One of the big reasons why marketers love Facebook Audience Insights is its ability to connect with super-specific groups of people. Since Facebook has such a gigantic user-base (around 2.45 billion in 2019), the tool has no trouble finding the right people for your business.
Marketing works best if you know who you want to market to. Unfortunately, many seem to forget about it or simply ignore it which sucks because finding relevant audience is not that hard. All you need is a little understanding and the use of the right tools.