All chatting app first of all increase their active user so more user use app for more time.
When millions of user use the app, then they collect the user data and analyse it in people manager which can Be use in marketing,
Chatting app company may sell user data to advertising company or rent it and make money.
Let take example of whatsapp, they collect your data like what you like most, what kind of group you belong to any many more thing. So later they (facebook) show you ads.
Like if any advertiser demand that their ads must Be show to iphone user, so whatsapp know who are using iphone .