Why is the solar cooker box covered with a glass sheet?

Solar cooker box is used to cook food with help of sunlight. The sunlight when it shines on the solar cooker the food inside gets cooked due to the absorbed heat from sun rays.

To avoid the escaping of the heat from the solar cooker, the solar cooker is covered with glass as sunlight can pass through glass effectively but the heat inside the cooker will not be radiated out.