There are many doors in a home that may not regularly get opened. It's not that there are skeletons in your closet, but there may be tools or seasonal decorations in there that make opening that door a less than regular event.
You may also have company or guests who could end up opening up your closet doors. Your guests may not be snoopy per se, but they might be looking for a bathroom and find themselves staring down the stairs to an unorganized basement. And if you have younger children,the last thing that you want is for them to accidentally open a door to a room with sharp or non-childsafe tools and chemicals.
These doors give you the opportunity to add a little variety to your home's design scheme. By choosing a richly patterned or beautifully colored fabric to dress your doorway, you are changing the entire design aesthetic of a space by simply adding a little diversity.
You can hang a window treatment over the frame of the door and keep it drawn, letting it known that this isn't the door to a bathroom or kitchen but instead just to a closet or basement that needn't be opened. Instead of putting up a do not disturb sign, hanging a a nice sheer drape or a velvet blackout curtain can be decorative and send a message.
Window treatments are so versatile, and many of their uses have nothing to do with the window itself. Utilizing your favorite fabric to add a dash of flavor to the overall design of your interior is also a functional way to establish boundaries.