Hi bm011.
Welcome to our Mattress Forum! 
Placing the mattress on the ground would reduce the ventilation of the mattress and especially in humid environments can increase the risk of moisture in the mattress and lead to the possibility of mold and mildew and even an increased dust mite population in the mattress. While the risk of this is small for a short period of time and also smaller with latex and other “treated” materials or foams … bearing in mind that mold issues develop well before they are visible I personally prefer options that have the least risk and promote the ventilation of the mattress.
There’s much more information about the functions of foundations, active slat systems and actual box springs in the foundation thread here .
I hope this helps
Phoenix