When Apple made the switch from 30 pin to lightning connectors, a lot of iPod docks became effectively obsolete overnight. Obsolete, that is, unless you’re willing to hack them to take other music sources, such as turning them into Chromecast speakers.
It’s a neat enough looking hack, albeit (as Hack A Day notes) somewhat lacking in documentation. There are undoubtedly easier ways to stream your music around than this hack, but it’s at least making use of a dock that might otherwise be gathering dust.