This read-write value names the ifIndex of the physical interface, or another virtual interface such as an Etherchannel, that a Virtual Interface Group is bound to, with zero meaning currently unbound. An admin cares because binding is mandatory for the group to become operational, so a zero value here explains why a configured Virtual Interface Group isn't passing traffic yet. For example, setting this to the ifIndex of an Etherchannel would bind the group to that channel, making it operational once the bind succeeds.