With unicast, the node originating the stream would have n times the traffic, where n is the number of devices pulling the stream. Further from the originating node in the network topology, each edge would transport as many times the traffic as there are listeners whose traffic travels along that edge. That seems very significant.
With broadcast, all the nodes and edges transport the stream only once, but you can only distribute the stream in the same (L3) network, and within that network you transport the stream even through L2 nodes and edges (i.e. network switches, cables, Wi-Fi channels) where there are no listeners at all. You could in theory repeat those broadcast packets into other networks, but you need to set that up more explicitly, and then every node and edge in that network gets the traffic, too.
With multicast, devices and effectively segments can subscribe to the stream, and all nodes and edges transport the stream at most once, and they can do so across network boundaries while retaining the same property, and using a standardized mechanism that is designed to make the traffic only go where it needs to go as much as the topology allows.
With broadcast, all the nodes and edges transport the stream only once, but you can only distribute the stream in the same (L3) network, and within that network you transport the stream even through L2 nodes and edges (i.e. network switches, cables, Wi-Fi channels) where there are no listeners at all. You could in theory repeat those broadcast packets into other networks, but you need to set that up more explicitly, and then every node and edge in that network gets the traffic, too.
With multicast, devices and effectively segments can subscribe to the stream, and all nodes and edges transport the stream at most once, and they can do so across network boundaries while retaining the same property, and using a standardized mechanism that is designed to make the traffic only go where it needs to go as much as the topology allows.