Many device discovery protocols work by sending out broadcast or multicast packets (either to announce themselves to devices who might be listening or to request devices to send them data). These packets are expected to go out to either everyone on the same layer-2 network (the broadcast case) or everyone who has subscribed to a particular multicast address (the multicast case).

In addition to device discovery, these are frequently used for heartbeat messages to indicate that you are still alive (for high-availability protocols like VRRP).

Thanks! That's helpful/interesting and makes sense

One common use case is multicast DNS (https://en.wikipedia.org/wiki/Multicast_DNS) which uses multicast for individual hosts to publish services available on them to other hosts on the network without needing a dedicated DNS server.

You can send data to N hosts by only sending it once, rather than N times.