WhatsApp lets people find each other with phone numbers instead email addresses. This has a profound difference in real-world usability because it decreases friction.
Before WhatsApp, people texted each other using cell companies' SMS service.
Why did Person A (who already had an email address) send a SMS text to Person B (who also already had an email address) to chat?!? Because both people already had each others' phone number in their smartphone's address books. Recording each other's email address is less likely so it won't be used as "ids" to chat -- especially between friends & family.
To wit, my mother has already memorized my brand new phone number that I've only had for a few years but she doesn't know my email address at all even though I've sent it and re-sent it to her multiple times. To normal people, the phone numbers are more sacred than email addresses. WhatsApp was a continuation of the conveniences of SMS -- without paying 10 cents per message to the mobile phone carriers.
Friction from cognitive overhead is a big deal in technology adoption.