[danesparza]

Aside from Pig Butchering (see other comments) this also verifies a number is real or in-use when somebody replies.

[fy20]

You can verify a phone number is real and active without needing them to reply, or without someone even receiving a notification.

1) SMS supports receipt functionality, which means the sender can be notified when a message is delivered to a handset. This happens at the carrier level, so you can't turn it off.

2) SMS supports 'silent' messages which are a different message class that is not shown to users.

[kube-system]

True, however, "delivered to a device" and "there's someone reading and replying at the other end" are two different confidence levels of "active".

You could, for instance, be texting an unattended device.

[toast0]

Receipts are unreliable. Carriers and aggregators regularly filter or fake them.

Receiving a receipt is not a real indication of delivery, and absence of receipt is not a real indication of absence of delivery.