My understanding of this setting(Multicast Rate) is to tell the router how strong a wireless connection a client needs to have in order to receive multicast packets. If a client has a weak wireless signal, you might want them to only be sent traffic that they requested, instead of also including multicast packets.
If I had a streaming TV signal, I might change the Multicast Rate to something higher so as not to bog down the weaker connections.
This would be why Auto is not an option.
Edit: My earlier post confused Broadcast (ie. mDNS) and Multicast. Here is a helpful link on that: