>both people already had each others' phone number in their smartphone's address book...
Look if it is saved in the smartphone's address book, why does it matter if it is email or a phone number. I bet that people don't actually memorize even the phone numbers of people close to them these days.
>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?!?
I think it is because we didn't have near universal internet access in smartphones for a long time. If Whatsapp (or something like that) was a little bit late to appear, and the tech crowd was actually a bit more smarter, things like Delta Chat had a much better chance to be in Whatsapps current place.
>Look if it is saved in the smartphone's address book, why does it matter if it is email or a phone number.
Because when people interact with each other in informal situations, it's the phone numbers that are shared. Not email addresses. Thus, email addresses are often not in the Contacts app at all.
Look if it is saved in the smartphone's address book, why does it matter if it is email or a phone number. I bet that people don't actually memorize even the phone numbers of people close to them these days.
>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?!?
I think it is because we didn't have near universal internet access in smartphones for a long time. If Whatsapp (or something like that) was a little bit late to appear, and the tech crowd was actually a bit more smarter, things like Delta Chat had a much better chance to be in Whatsapps current place.