Read receipts are a useful tool, but plenty of people don’t like them – they can be intrusive. Who wants to broadcast when you’ve read a message?
There’s an underlying expectation of a reply, too. It’s not something you want to broadcast all the time.
To turn off read receipts in iMessage, simply head to the Settings app in iOS, then scroll down and select the “Messages” button. There, you can select to turn off read receipts – and change a few other settings if you’d like, too.