This is the master switch for IGMP snooping/processing on the unit, which is what lets it actually track which multicast groups are being requested downstream rather than treating multicast like ordinary broadcast traffic. Enabling this is a prerequisite for the per-group IGMP monitoring table elsewhere on this device to have anything meaningful to report at all. For example, turning this on before rolling out an IPTV multicast service is what allows the unit to later report per-group membership state instead of just blindly flooding every multicast stream.