They have the room, the gear, the ear and the experience. Mastering has gotta be up there with the most misunderstood and overlooked aspects of production.

100%, the artist I work with usually write, produce and mix their own songs and uncertainties introduce themselves in every step of the way. When I get to work on the track, I don't have any knowledge of any previous version or vision and most importantly, no emotional connection to the sound so I can make less biased calls on what to do to get the song to the desired place in a competitive way