A bed frame may seem like an unnecessary expense, especially when you consider that people in Japan have successfully avoided using them for hundreds of years, and continue to do so. Unless you're on a student's budget or plan to move within the week, though, it's best to keep your mattress in a frame. The space under your bed allows energy to circulate, provides stability, and is useful for storing pillows and blankets.
Support for the Box Spring
By itself, a mattress doesn't always supply the support you need to get a good night's sleep. The mattress needs to rest on a box spring, and many manufacturers make that a condition of the warranty that they supply with their products. A box spring is constructed inside a wooden frame, but it's one that has no legs or feet, and if you place it directly on a hardwood floor, it will scratch the finish. Moreover, in the absence of a frame, there is nothing to stop the mattress from sliding off the box spring.
Feng Shui Considerations
According to feng shui, the ancient art of space harmonization, your bed should be raised above the floor to allow chi, or energy, to circulate. If the bed is on the floor, not only does energy stagnate, which can affect your sleep patterns, you are also in a position that makes you more vulnerable to drafts and cold air, which tend to collect in the lower parts of a room. Putting your bed on a frame gives the bed a feeling of permanence and stability, especially if you arrange it so the headboard is against a solid wall.
Mold and Insect Control
It's customary in Japan to take advantage of the comfort of tatami floors by laying out a futon, but the futons used for this purpose are small enough to take up in the morning and air out. The same isn't true of Western mattresses; they are too large and heavy to move easily, so they usually stay where they are. Because of the lack of circulation, the space under the mattress becomes an ideal environment for mold and insects. A frame that keeps the bed off the floor prevents this problem, and it also makes it easier to get into and out of bed.
Storage Space -- but Use It Wisely
Lifting your mattress off the floor by putting it in a frame creates a natural storage space, but it isn't an extra utility closet. Feng shui counsels to avoid storing old clothes, books and letters or metal objects under the bed; these affect you in a negative way by blocking the energy that circulates around the bed. The best use for the storage space under the bed is for extra pillows, blankets and other furnishings you use in bed. Since you won't need to put these in a closet, you'll have extra room there for everything else.