The need for a glass top for the tank is dependent upon the type of lights used, the location & type of the tank, and the tank fauna's propensity to escape.
If the tanks lights are on tank top stand, or hung type a glass top is necessary to enclose your tanks top for prevention of introduction of foreign objects, and to prevent any of the tanks inmates trying to escape the tank's confines.
If the tank is located in a place where the tanks temperature is difficult to maintain above the normal ambient temperature, a glass top is indicated and the contrary is also true - where the ambient temperature tends to be higher than that of the tank, a glass top is contra-indicated.
Tanks with hoods which are capable in preventing any introduction of foreign objects, or escapees from the tank, necessarily would not need a glass top for any other purpose than preventing fast evaporation. Tanks without such hoods would naturally need either a net or a glass top, especially if its fauna are escape artists.
