Exactly. People started using and are still using Whatsapp because it allows global, free communication. When it started getting popular, greedy service providers were still charging several euro cents for a few bytes of data sent via SMS, even to the same network, of course people went for the free alternative (I actually installed Viber before WhatsApp).

Nowadays it still has the advantage of being free when communicating abroad. I travel a lot, and calls from foreign countries, sometimes on different continents, are expensive and low in quality; a local data SIM on my dual sim phone allows me to call friends and relatives at home with the same ease as a normal phone call and without crazy costs.

It used your existing phone number as your identifier

Wait, people think of that as an advantage? For me, that has always been the reason I have refused to use it. It's also why I never even tried Google's Allo despite being a big fan. A chat application that isn't using the phone network shouldn't be tied to your phone number. It makes cross-platform support extremely awkward, and that's noticeable in how poor What's App still is for cross-platform use.