I thought he/she would have to fine-tune each track individually and then again as a whole, which would be easily done dry.
Well, if you by "track" mean a series of mixdowns of different songs (tracks) meant to be presented together, like on an Album or an "EP", this is ideally what a master engineer does. Fine tuning already balanced stereo mixdowns of "songs" or tracks. I guess what he or she specifically does with a mix varies, somewhat at least, with material.
Personally I think a mixdown should already be as "loud" as you want it, with the crest factor on each track having the needed perceived volume together with any parallel compression — which I love to use — and so on.
The fact that what I turn over for mastering typically aren't peaking over -6dBFS is to make some head room for the mastering process.
My main focus is to make my mixes sound as balanced and great as I can and to put across the emotions I want my music to potentially communicate. That's what I rely on the master engineer to understand — I'd assume each one does this in her/his own way — and enhance into a master worthy of being released.
I know machine treatments are on the go now, so that's an alternative for some stuff. I'm on the fence so far for that.