Well, the size, for one thing: a too-big walk in closet has room for clothes, books, and a desk. But the key here is the window, which turns what would otherwise be a small, stuffy room into a pleasant place where it's possible to focus attention on the work at hand and escape the din and clatter of a busy house.
When designing, it's easy to imagine very constrained uses for rooms: kitchens are for cooking, bedrooms are for sleeping, closets are for keeping clothes -- and the names we use for these rooms reinforce our tendency to think of each room as serving a single purpose. But something as simple as adding a window to a closet can open up a world of possibilites.
We're wondering if there's a space in your home that you're using for something other than its intended use. Perhaps you've turned your garage into a craft room, or a spare bedroom into a place to do laundry and sew?