Knowing someone's phone number means you can reach them on WhatsApp. At least, that's true for 2B (out of 5B) of phone numbers, which is likely to be 1,000 times the number of RCS users.
The second and most important advantage of WhatsApp is that you don't have to maintain an address book anymore. People change phone numbers and the new number is automatically reflected in WhatsApp. It's quite likely that address books on phones are outdated and thus stale.
> Knowing someone's phone number means you can reach them on WhatsApp
...that would be super useful if I had WhatsApp installed. It also means that WhatsApp is tracking the phone number of all the people and doing selective (assuming only your friends can see it) publishing of it. I guess that's usually okay, but it also kinda wierds me out that it leaks info that way.
My phone number is already managed by me, I've given it to the people I want to have it. This really just limits any WhatsApp messaging I would to to entities that I want to have my phone number. It's probably much easier to block WhatsApp accounts (and attempted messages) than calls (phone calls which are synchronous communication have a much higher urgency to me than some random phone ding that's async).
The second and most important advantage of WhatsApp is that you don't have to maintain an address book anymore. People change phone numbers and the new number is automatically reflected in WhatsApp. It's quite likely that address books on phones are outdated and thus stale.