When installed, the inbox from the core Instagram app disappears and is replaced by the new standalone app. "We can push the boundaries to create the fastest and most creative space for private sharing when Direct is a camera-first, standalone app". Instead, you'll have to DM your friends through Direct.
That makes sense. Just look at Facebook Messenger, which was first released in 2011, and made the default mobile messaging experience since 2014.
So why is Instagram experimenting with a standalone messaging app?
According to The Verge's Casey Newton, the raison d'étre behind this app is that Instagram believes messaging can never truly shine when it's tethered to something else. Along with your regular feed, there's the Instagram Explore Tab for finding photos the app thinks you're interested in, a Stories feature taken straight from Snapchat, live streaming, and the ability to send direct messages to other users.
The Direct app, in its current state, is simple and straightforward.
Focused on image sharing, Instagram Direct opens directly to the camera so sharing can be easier than ever before. You'll have the option of taking either a photo or video and adding your own effects and filters, with some even being exclusive to the app including a superimposing mouth and censor bleeps at random times. For starters, it's another big step in Instagram's metamorphosis from photo sharing site, to broader social networking platform. The page to the left is where you'll find all of your account/app settings, and the one on right is home to your inbox of ongoing conversations. You can even go further and swipe to the right once more to open Instagram.
When Facebook split Messenger off into its own standalone app, it had about 500 million monthly active users.