There are a few points I can see

1. Console-like living room ready experience. It's surprisingly hard to get a PC made with off-the-shelf parts to integrate cleanly with a home theater system (think features like HDMI CEC, One Touch Play, etc). A custom SoC can solve this, something we are seeing Valve also do with the Steam Machine.

2. As the target hardware for basically all Xbox games, end-users who don't want to fret over system specs can easily just buy this and know they are getting the intended experience.

Whether that's enough to move units remains to be seen.