multicast using MBGP

Hi,

I would like to know why MBGP is used for multicast ?
I came across some document which says that it is used for RPF checks
and
MP_REACH_NLRI is being used to carry that info, but I would like to
know why
the regular BGP cannot be used for this purpose. I know that MBGP is
not a
multicast routing protocol, but would like to clarify its role in
multicasting.

Is it used in conjunction with BGMP ? If yes, please clarify.

Thanks,
asm

.