Maturing:
I find that macarons taste the best after they've rested in an air tight container in the fridge overnight, like the cookie above. The filling has time to flavor the shell slightly and the slight humidity improves the overall texture of the cookie. This is called maturing. The higher the moisture content of the filling, the faster the shell will mature. Ganache filled macs will mature faster than butter cream macs, etc.
Of course, overly high moisture fillings do not lend themselves to maturing as they will reduce your macarons to a blob of goo.