Mastering is more than “turn it up”.
The native idea of mastering is to finalize the mix, to embed it into the nowadays sound-environment and to optimize the sound on each and every speaker situation.
It is possible to add punch, warmth and transparency to the material, or to make it sound bigger and wider.
But it isn’t always clever to bring the mix to the edge of loudness and to kill all the dynamic of your music. Not at last emotion is carried by dynamic, and zero-dynamic-productions will tire up your ears after a while. Nevertheless it is necessary to limit the dynamic in a reasonable amount.
In the end it’s up to an expired mastering engineer to hear what is missing in the mix and use his know how to do what needs to be done.