Although people typically install it on their phones to chat privately with their Facebook friends, Facebook also uses Messenger to bring features and capabilities that might not make sense, or even be possible, as part of the main Facebook service. And of course, a separate app gives Facebook even more advertising and other moneymaking opportunities.
For that reason, Facebook is pushing users to download the app, even though it takes up valuable storage on the phone. The company recently started blocking access to Facebook messages from mobile web browsers on Android phones in some markets. The ban will extend to iPhones as well, though Facebook isn't saying when.
Before you complain, consider what a separate Messenger app offers beyond simply typing words and sending emoji back and forth: