What should be the first target for Mutalisks?

In ZvZ what should be the primary goal of mutalisks? Recently I have been hearing it is best to target overlords out on the map before trying to poke into your enemies bases. Logically, I am thinking it would be best to see if any damage can be done to the base, particularly the mineral line, before proceeding to systematically kill the overlords on the map and gain map control. What is the logic for professional players to go after overlords first? What is the best use of the investment?