Multicast routing over a GRE tunnel

Consider a network of routers with multicast routing turned on. Somewhere in the network there are a pair of adjacent routers with two equal cost paths between them. You display the config on the routers and see the following -

On Router1


interface Tunnel0
ip address 6.1.1.1 255.255.255.0
ip pim sparse-dense-mode
tunnel source Ethernet0/0
tunnel destination 5.1.1.1
!
interface Ethernet0/1
ip address 2.1.1.1 255.255.255.0
!
interface Ethernet0/2
ip address 3.1.1.1 255.255.255.0



On Router2


interface Tunnel0
ip address 5.1.1.1 255.255.255.0
ip pim sparse-dense-mode
tunnel source Ethernet1/4
tunnel destination 6.1.1.1
!
interface Ethernet1/1
ip address 2.1.1.2 255.255.255.0
!
interface Ethernet1/2
ip address 3.1.1.2 255.255.255.0
!



That is, there is a GRE tunnel between the two routers and multicast is being run over the tunnel rather than directly on the physical interfaces between the two routers.

Why would such a thing be done between directly connected routers both of whom are multicast capable? Is that tunnel superfluous or is it actually being useful in some manner?