For which two purposes is an IP address configured on a tunnel interface?

Use of dynamic routing protocols: An IP address is needed on the tunnel interface to participate in dynamic routing protocols (like OSPF, BGP, etc.) over the tunnel. This allows the firewall to advertise routes and receive updates over the tunnel.

Tunnel monitoring: The IP address on the tunnel interface can also be used for monitoring the tunnel's status. Tunnel monitoring (such as IPSec tunnel monitoring) requires an IP address on the tunnel interface to check the health and availability of the tunnel.