About mDNS

Q: What is it?
A: A network service announcement/discovery system (a "DNS" for services)
See http://en.wikipedia.org/wiki/Zero_configuration_networking
Q: What is it good for?
A1: Announcing network shared services - printers, files, music, web interfaces, etc..
See http://holyarmy.org/2008/01/advertising-linux-services-via-avahibonjour/
And https://bbs.archlinux.org/viewtopic.php?id=48558
A2: AirPrint for existing printers!!! :D