[0] https://cdn.nieuws.nl/media/sites/305/2016/05/14195316/Borde...
If I phone a tradesman, e.g. a plumber, and they ask for a picture of a job, the photo will be sent using whatsapp.
It's ubiquitous and it's assumed that if you're have a phone you have WhatsApp.
The only people I can think of that don't use it are my wife's grandparents, they use Facebook messenger for that purpose.
Back when people were switching to signal a lot of people cited the fact that signal needed a phone number as a reason not to use it, instead recommending session, threema, matrix.
But if signal didn't tie your account to your phone number there's no way I would have convinced any of my contacts to switch.
Having to exchange username or user IDs wouldn't have cut it, with whatsapp the service is deeply associated with you phone number and that's what people want.
It's free advertising in two ways. One is for Facebook/WhatsApp (the use of these signs further normalizes WhatsApp and strengthens the brand), and the other is as a friendly gesture to criminals: here live people who can't afford fancy alarm systems and private security subscriptions, but can afford plenty of easily stolen devices and other loot.