The main reason that you will see people do this is that the base layering (which would be the tank top in this case) is going to wick sweat away from your body. So, in essence, even if you did not have an outer layer that was breathable this would at least help a bit. Then again, you should really wear clothing that is all breathable when you are working out. This is simply a solution if you feel the need to add more layers (and this can also come into play if you want to do some exercising in the winter or during colder climates).