Sub Floor ventilation allows your sub floor to breathe by having cross ventilation, reducing the amount of moisture buildup and therefore reducing the risk of Mould.
The first thing you should do if you are worried about your sub floor is to call a professional to check your cross ventilation, and to see that there is no water leakage into the sub floors of your home. A sign that there is something wrong with your sub floor is if there is dampness, mould, and rooms that have a musty smell, along with other visual indicators such as convexing floorboards.