OSPF Virtual Link Lab

For the OSPF Virtual Link Lab, I only created a virtual link between R1 and R4, as per the requirement ‘Ensure that full reachability is maintained even if R5 loses its link to the DMVPN cloud’.

However, the solution also has a virtual link connection between R4 and R6, which I can see does mean R7 and R9 will reach parts of the network more efficiently by going via R6 instead of R3, but aside from that, I can’t understand how it contributes towards ensuring full reachability if R5’s tunnel goes down.

Can someone please explain, thanks.