Why does an RODC have two inbound connection objects?

This is because File Replication Service (FRS) requires its own pair of connection objects in order to function correctly. In previous versions of Windows Server, FRS was able to utilize the existing connection objects between two domain controllers to support its replication of SYSVOL content.

However, because an RODC only performs inbound replication of Active Directory data, a reciprocal connection object on the writable replication partner is not needed.
Consequently, the Active Directory Domain Services Installation Wizard generates a special pair of connection objects to support FRS replication of SYSVOL when you install an RODC. The FRS connection objects are not required by DFS Replication.