Supposedly, it was designed for Tablet PCs, where fingers would be the primary interface device.
Microsoft assumed they could get away with "One Interface For All", that happens to be 'optimized' (to use the term loosely) only for tablets.
It didn't work. The UI is full of design flaws, that they are slowly correcting, with each patch that comes out.
For example: For Win 8.1 Update 1, they finally have right-clicking menus that show up where you right-click the mouse, instead of bringing up the menu in another spot (icons on the bottom of the screen), on the Start Screen.
It made no sense, from a design stand point, for the user to right-click in one spot, and have to move the mouse to another area to get to the actual menu they were bringing up.
Eventually, they will re-introduce the Start Menu, I think.
The sad thing is that all of this could have been avoided, if they hadn't brushed off the science of practical psychology and interface design so quickly. They pushed Win8 out the door, before it was barely dressed and didn't even have time to brush its teeth!