alt Hacker NewsWhatsApp has replaced the PSTN in many countries. Without it, you cannot contact businesses; they simply do not answer normal phone calls. Also, booking accommodation over the internet may require WhatsApp in order to receive self-check-in information, and if you manage to communicate to the accommodation that you don’t have WhatsApp, it may simply cancel your reservation unilaterally.
Even where its not required, it can be a lot cheaper (e.g. for international calls) and has features (conference calls) not offered by consumer phones.
Whatsapp's success has partly been driven by telecoms companies over-charging.