So you don't need to memorize and hard-code IP addresses for your network services.
* router.home.arpa * nas.home.arpa * printer.home.arpa * htpc.home.arpa
* router.home.arpa * nas.home.arpa * printer.home.arpa * htpc.home.arpa
mDNS is probably sufficient for most people and you don't need to setup old school DNS on a local network. (But if you do, a purpose-built TLD like home.arpa is a good choice.)
With all new gTLDs it’s easy to have a collision when someone buys the “router” TLD.