The Box Spring for a mattress has 3 different purposes but the main purpose of the box spring is to add support for the mattress.
This prevents the mattress from falling through the bed frame or slats and holds the mattress up so that it doesn't sag.
The box spring helps provide a more comfortable mattress and if you do not want the box spring and just want or need a mattress you could put a piece of plywood underneath the mattress to hold it up.
But the box spring for mattresses also has a few other purposes and points which are to help raise the height of the mattress to make it easier to get in and out of bed and to also absorb any shock to the mattress and helps prevent wear and tear of the mattress.
I've used mattresses without box springs and it's been just fine but I put a piece of plywood underneath the mattress for support.