Well it started because I bought a lot of props when I was on holiday in France, food equipment, plates etc, are just so much cheaper there.
I've also got a lot of other things like background papers and materials that I needed to keep somewhere.
Funnily enough, my props cupboard ended up being in the bathroom though - we have a huge airing cupboard in there and it was just sitting empty, far more space than is needed for a few towels, sheets etc, so it became my props cupboard.
When it gets a bit full though we're lucky enough to have a storage facility in the basement of our flat building - I keep meaning to get some shelves to put in there to turn into a much more organised space, and then I'd probably keep them all in there...
My "studio" is the most amusing thing - an ironing board next to the window