Box springs act as the foundation and support system for many types of mattresses. In fact, when you go shopping for a new mattress, a box spring is often sold along with the mattress as a complete set. Makers of mattresses and those selling mattresses may insist that you need a box spring, but whether or not you do depends on a number of factors.

Type of Bed Frame

Consider where the bed will be and what type of frame will be used. If you are using a traditional four post bed frame you will need a box spring. This is because the mattress is soft and will fall through the space in the center of the frame. With wooden slats frames that have only a few slats, areas of the mattress may sag through the spaces between these slats causing discomfort and mattress wear. A box spring will hold the mattress up and keep it flat on the frame. If you have a platform bed, usually a solid piece of wood or a frame with tightly spaced slats, a box spring is not needed, because the firm support of the wood or closely placed slats keeps the mattress from sinking or sagging.

Preffered Bed Height

Determine your ideal bed height. If you do not mind sleeping close to the ground, you do not need a box spring. If you prefer a higher bed, or you have trouble getting up out of a low position, a box spring will add needed height to your bed.

Matress Design

Look at the mattresses and the box springs sold with them in the store. Some mattresses are designed to work specifically with the box spring sold along with them. Skipping the box spring or using an old box spring may not allow you to make the most of the support system design of the mattress. Soft and ultrasoft mattresses may require the specially designed box spring to prevent sinking and sagging. Also mattresses that can be flipped to reduce wear and tear may become damaged if flipped onto hard wood platforms or wooden slats. The box spring in this case provides a surface that will not tear the mattress. When using sheets, the space between the box spring and mattress can help hold the sheets in place; especially ideal if you prefer your sheets tucked. Without a box spring, the sheets may slide off frequently or become caught on the frame or slats.

Source:

The Better Sleep Council: Do I Need to Buy a Boxspring?

Sleepy's: Mattress Shopping Tips