Tablet gaming sucks. If I had $400 to blow, I would gladly buy an XboxOne or PS4 over a tablet or Nvidia Shield.
Oh wait I did! Man I'm loving Destiny beta!
The SHIELD products are designed to be a high powered mobile gaming device and an extension of your gaming PC. With SHIELD you can play classic game emulators for systems like NES, SNES, Gensis, Playstation 1 / 2, N64 and more... while also being able to play all android games with a physical controller, at any time or place.
Also, with a wifi connection, or on your home network, you can stream PC games from your PC to the device. So you can play Skyrim while you're waiting for a plane at the airport, or game through Boarderlands 2 while you're on a break at school or work.
The SHIELD tablet can attach a controller, like Game Grip STG, or it can be used without one for normal tablet/touch screen uses.
But this is stuff that appeals to PC Master-Race gamers. Dirty console peasants need not apply.