This new app is the stock SMS/MMS handler in Android 5.0 Lollipop, but also can be downloaded to other devices with SMS/MMS capabilities, provided they run Android 4.1 or higher.
So, what's special about Messenger? Honestly, as texting apps go, not a lot. You can share pictures and videos from the chat dialog, search your contacts and conversations, block SMSs if you need to, send audio messages, color messaging threads, and send emoji, of course.
But seriously, why does Messenger exist when Hangouts supports SMS/MMS integration? Probably because there are a whole lot of people out there who have no interest in using Hangouts, because honestly, the Hangouts app is kind of slow and annoying sometimes. Considering Hangouts replaced the stock SMS app in Android 4.4, this is quite a backpedal from Google.
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