[c-nsp] Why does PIM enable IGMP?

[John Neiberger]

IGMP is enabled automatically when you enable PIM on an interface and
I can't figure out why. I've talked to a few other engineers and no
one can thing of why IGMP would need to be enabled on a
router-to-router connection running PIM. My only thought was that it
was a leftover from PIMv1, which used IGMP type 14 packets, but that's
a total guess.

Do any of you happen to know the answer to this piece of trivia?

Thanks!
John