The role of the multicast router is that it “polls” (general queries) if there are any receivers still interested in receiving multicast traffic for certain groups. Receivers will always respond to this.
IGMP snooping requires this “poll and replying” to work correctly. Otherwise it will be unable to learn all receivers in the network. By default, the router will send these general queries. If there is no router, we can also configure the switch to send these general queries (as demonstrated in this lesson).