Shortest answer possible, juno: The product you linked to allows one to access their music and videos (stored as digital files) locally (via a local area network in one's home) and remotely (via a wide area network, i.e. the internet) by means of Microsoft RPC, TeamViewer software (free), or an internet accessible app for software music player Foobar 2000. I've had a Windows-based setup of this sort since 2001 and have enjoyed it thoroughly on nearly a daily basis since. The device enhnaces the playback experience by removing PC fan noise which could interfere with soft passages/low-volume listening, provides high-speed transfer of files between itself and a client/remote computer or device (tablet or phone) via 802.11 n wireless or gigabit ethernet. It allows allows the user to watch video files on a high definition computer or television via the listed HDMI connection.
So, in summary, the MP-Berry V2 music server allows its owner to access their music and video files in or away from home via the internet. While at home, you (and your family) can access the aforementioned files from other computers/devices throughout the house on your wireless or wired network. Away from home, those same files can be accessed and played back by means of Microsoft RPC, TeamViewer or Foobar 2000's app on your phone or tablet. In regard to the MP-D2 DAC add-on option, it personally strikes me as overkill. I don't know if you or your wife are audiophile purist types, but something telling me you won't be listening to high resolution formats with sampling rates out to 384 kHz or multi-megabit DSD files. If I were in your shoes, I'd buy the base unit as presented.