According to Facebook, they're currently testing the ability for people to easily bring all their conversations, from SMS and Messenger, to one place.
It’s really a simple way to access and respond to all your SMS messages within the Messenger app, alongside all the other features that Messenger offers.
Also, Messenger now supports multiple accounts at once, so users don't have to switch profiles by signing out of an account.
Facebook had always touted Messenger as a cross-platform messaging application, while trying to make this happen through integration with other services, allow you to use Messenger for all your conversations.
The importance of messaging for businesses to communicate with their customers, makes multiple account support very crucial, hence the launch of the feature on Android to enable users to switch profile from a single account.
If you want to try out the new features you can get latest version of Messenger App in the Google Play Store, then go to the Facebook App settings and see if you have Change SMS App as an option.