This is a standalone sms app. Probably part of AOSP. Hangouts requires a Google account. Which is not idle for everyone. And also for markets like China. Google needs to provide a dedicated sms/mms app for these markets/consumers.
Stay in touch with friends and family. Messenger from Google is a communications app that helps you send and receive SMS and MMS messages to any phone. You can also send group texts as well as your favorite pictures, videos and even audio messages to your contacts.