More likely it's about getting more immediate attention from push notifications.

If you only ever use chat in the browser, you only see messages when you actively check it. With the app installed, you get a giant bubble that you can't ignore for every message. FB wants to be the messenger that people respond to immediately.

For me, I actually like the messenger bubble UI, and love that it's not built as a gateway to the massive time sink of the rest of Facebook. But I think it's clear that omnipresence is the goal here, and we have no reason to believe that it won't become a funnel to the rest of Facebook in the future.