What is the use of SMS receipt function?

When the other party receives the short message you sent, the system will send you a receipt in the form of a short message, indicating that it was sent successfully. Generally, the receipt will indicate the sender and the specific sending time, so that you can confirm that the message has been received by the other party.

This function is optional.

And receiving the receipt doesn't mean that the other party has seen the message, just open the phone and received it.