This read-write InterfaceIndex is a filter criterion object: writing an interface index here selects Neighbor Binding Table entries anchored to that interface to be removed when the clear action is triggered. An admin cares because it lets an admin clear every binding learned on a specific port at once, useful after disconnecting or replacing whatever device was attached there. For example, setting this to port Gi1/0/20's ifIndex before triggering the clear action removes all bindings anchored to that port in one operation.