The idea behind the iPod dock was a good one, at that time (barely less than a decade ago) almost everyone who listened to digital music had to be doing so on an MP3 player of some kind or an Apple iPod, the iPod was the most popular device and iTunes was a popular way to collect music.
By plugging your iPod into the dock you could - using the SONOS controller (remember them) select and choose any music from any docked iPod in your SONOS ecosystem. The problem with iPod docks (all of them) is that Apple are constantly changing interfaces, plugs, adaptors and software so eventually they just stop working properly.
Most of the people still using a SONOS ipod dock in 2018 will be using it to run music off an old iPod they had lying about, it’s easier to store music on iTunes or use something like SPotify and stream directly from the cloud nowadays BUT some places don’t have internet, so an old ipod loaded up with your favourite music could still be useful, not for long though!