It's also the cheapest. Because its free with WiFi, and cost less than US$1.00 per day with a data plan. Compare that to phone calls that cost about US$ 0.15 per minute or SMS that cost US$ 0.06. It may not seem much, but when you factor the local minimum wage in urban areas: $1.87/hour or US$ 0.03/minute the difference becomes significant.
Perhaps the closest alternative here would be Telegram. It has niche users like government, NGOs, political organizations and group chats for people with similar interests (like Facebook groups are used in some countries).
I guess, WhatsApp has become like our tropicalized version of WeChat.
Its used to chat with friends, family, group chats (leaving a group chat unannounced is a big faux pas). Almost every class gets a WhatsApp group for coordinating coursework.
Most stores and restaurants, big and small, accept orders via Whatsapp and that way one can avoid the third party app delivery fees. It is also used to send bank transfer confirmations, and gps location pins to coordinate deliveries because our addressing system is not that good.
I even did most my last loan application via WhatsApp. And also discovered last year that the only way to contact one bank about an ACH transfer was via WhatsApp or by visiting a branch.
So basically, it's the most used and most affordable main way to reliably reach anyone or any company. From the largest bank, to the smallest mom and pop store. From my company's founder to the newest intern.
The only thing I still get by plain SMS is multi factor authentication tokens or the ocassional spam message.