@evan SMS is a best effort notification with no indication it was delivered. You can lose messages, or get them delayed by hours. Also, carriers charged 25 cents per message back in the day.

Whatsapp on the other hand shows you the message was delivered, it's almost always instantaneous.

Downside, you need data connection. Back in the 2010s in a trip in Scotland, whatsapp managed to receive and send its messages before android could update its icon to indicate there's an internet connection.