A common pattern we have seen in the early days of Aries agents is a user with a browser getting to a point where a connection is needed between the website's (enterprise) agent and the user's mobile agent. A QR invitation is displayed, scanned and a protocol is executed to establish a connection. Life is good!
However, with the invitation processes described in the Aries Connections protocol, when the same user returns to the same page, the same process is executed (QR code, scan, etc.) and a new connection is created between the two agents. There is no way for the user's agent to say "Hey, I've already got a connection with you. Let's use that one!"
Out-of-band invitations described in the Aries Out-of-Band protocol aim to solve the returning user problem.
When the user already has a connection with the Enterprise agent website, scanning a second QR code (containing a new Out-of-band invitation) will not create a new connection. Instead, a user agent will respond with the reuse message over the first connection referencing the 2nd invitation. This way the enterprise agent can identify a returning user (and log the user in). The enterprise agent will use the first connection for future communication with the user in the current session.
A sample application demonstrating a returning user journey with Out-of-band invitations can be found here