There is a lot of confusion these days about what makes a good box spring, and for good reason! Box springs have changed a lot over the years. In the past, box springs were made of wood and coils so that they reacted to the sleeper. For innerspring mattresses in the past, this helped cultivate a softer, more comfortable sleeping surface.
Nowadays, most mattresses don't have centralized innerspring systems that require this type of box spring. Pocket coils, latex, and memory foam beds don't require traditional box springs and actually they may harm the overall experience by causing extra sagging issues because of the less stable base. That's why today's box springs and foundations are solid, made of wood and metal slats that give a flat surface for the mattress to rest on. This gives an even comfort and extends the life of modern mattresses. If you have an older box spring with coils in it and are buying a new mattress, you definitely should consider getting a newer box spring or foundation. Also check your warranty for your mattress, it may tell you what's best.