Why is syrup added to the inside of a cake before frosting? Maybe the cream sticks better with each layer of cake?
Brushing a cake with simple syrup before frosting is done to prevent the cake from drying out too quickly, and sometimes to add flavour (if a flavoured syrup is used instead of just water+sugar).
The syrup is added to the cake before frosting so that is remains moist throughout. Also you can add flavours to your simple syrup to enhance the flavour like cinammon or star anise in banana cake and orange cake. For mango cake, you can simply add cardamom to syrup. Avoid using simple syrup if it is already moist and delicate. It would destroy the cake.