Broadcast traffic is spammed out every single port on all switches for the vlan that receives it. A lot of unnecessary traffic that the endpoint devices need to drop, especially if you just need to communicate between a handful of devices.
Multicast traffic is only sent out switch ports that are part of the multicast group. In other words, only the devices that need to receive the packet actually receives it. And it's very bandwidth efficient to send a single multicast packet to a remote router/switch that then copies that packet to all the multicast group members; compared to sending individual packets to each of the devices over the link.
For a CARP/VRRP consisting of only a pair of devices, then unicast vs multicast is pretty much a wash. The same number of packets are delivered for both setups, and most of the processing is done on the clients.
For a CARP/VRRP consisting of 3 or more devices, then multicast makes sense as only a single packet needs to be sent from the client to the switch, and it's up to the switch to do the copying and delivering to the rest of the group. Using unicast between 3 or more devices would require each device to send multiple packets (1 to each remote device) and for the switches to deal with all those extra packets.
Scale that up to dozens or more devices, and unicast/broadcast will bog down the network.
If only Apple would realise that broadcast-based protocols are the spawn of the most evil of demons and would move to multicast (or even unicast). My god is Bonjour horrible to manage, especially on wireless networks!!!!!
