This preserves the total count of Mode Configuration settings dispatched by the local entity over a now-expired tunnel's lifetime, per its description - relevant when this device had been acting as the VPN head-end assigning attributes to that client. It lets an admin confirm, after the tunnel is gone, whether the head-end actually sent the client its internal address during that session. A history value of 0 alongside a tunnel that stayed up for a while would be a clear sign the address-assignment step silently failed.