Correct support is an essential ingredient for a healthy body. A good mattress and foundation will gently support your body at all points and keep your spine in proper alignment, or in a “neutral” sleeping position.
When “test resting” a mattress, pay special attention to your shoulders, hips and lower back for pressure points, and/or pain. If possible, try to err on the firmer side. You can always soften a mattress with a topper, but it is impossible to make a soft mattress firmer.