Reasons for a boxspring:
1. To support the mattress over a raised open frame. Mattresses are engineered to breathe. Your body gives off dirt, dust, and moisture that would build up in the mattress over time if nearly one half of the surface area were blocked. Note that this is especially true in foam edge mattresses like Sealy Posturpedic and Spring Air Back Supporter. That dirt, dust, and moisture can cause unhealthy issues like mold and dust mites to increase to the point where they pose a breathing hazard. It’s worth pointing out that for times shorter than about one year, these effects are not hazardous.
2. To support the weight of your body. Mattresses themselves are not enginered to support your body. They are engineered to give you proper spinal alignment, and proper comfort. The box spring absorbs your weight, allowing the mattress to do its job more effectively, and having the mattress last longer because it is doing less work. Now, it’s arguable that a box spring, at 30% of the price of the mattress, allowing the mattress to last 50% longer is not that much of an economic incentive. (saving you about $35 per year)
3. To avoid “shock” associated with sudden impact. This is what the industry tells us, anyway. I am personally not convinced that a box spring is going to help someone who insists on jumping up and down on their bed, or having constant vigorous sex on top of it. That person seems beyond help anyway. I might add here that the common misconception that it is just a box is incorrect. Except for the very cheapest models, all box springs have working steel structure inside.