This iOS 10 setting was likely overlooked by many, considering all of the flashy new features Messages gained. Previously you could either send read receipts (read receipts tell the sender when you have viewed their message) to everybody or nobody; now you can set it on a case by case basis. Perhaps you want your significant other to know if you saw a message but not your boss. Set this by tapping on the info button in the top right corner of a conversation, then toggle on/off "Send Read Receipts."
How do I turn on individual read receipts in Messages?