Why is syrup added to a cake before frosting?

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.

Simple syrup add a great taste, but be careful not to add too much syrup. The frosty will collapse. Just add 1/4 cup to each layer of your cake prior of frosting. Wait for an 1 hour to absorb the syrup prior of frosting. This happened to me for adding to much syrup.