I had no idea what for....but I know I needed one badly! My CS education featured a lot of work on early Unix machines and the fridge size boxes we used were probably less powerful than the credit card sized RPI. I was also an early adopter of a variety of personal computers including an Altair, a Radio Shack TRS-80, a variety of z80 and 8080 based machines, and the list goes on from the antiques to my current iMac 27.
So, I had to have one. What to do with it? I already have the iMac, a Mac Mini, a laptop, a iPad, an Android Pad, and an iPhone so I am not short of electronics. I also have several media players so gen'ing up the RPI as a media player didn't sound exciting.
Then the Mars Rover landed and I had my idea! I have always been interested in the interface between a computer and physical devices so this was interesting. In addition I could do the development in Python learning another language on my route back to my past.