If your bed already has a built-in wooden slat, then a box spring is not necessary. However, you can still use a box spring to increase the height and sturdiness of the bed. Box springs provide more cushioning than slats, though. Slats can provide adequate support to the mattress even without a box spring. That being said, there are some slats that are designed to support box springs and not the mattress. This type of frame will have the slats fixed far apart from each other. For such frames, it is necessary to use a box frame.