I think you are confused about what mastering actually is.
It's not (necessarily) what you put on your master buss when mixing.
You as a mixer send a good sounding mix to the mastering engineer. He's gonna review it and treats it with processing as a whole. He doesn't have access to the individual channels anymore. As he listens in a Perfect environment he can spot problem which you as a mixer couldn't, maybe. This could be for example too much dynamics. Like when you're mixing loud all the time your drums will most likely end up too loud. Don't ask me why. The ME then maybe can press the drums a little bit back into the mix.
Compression isn't used evertime during mastering. And when it's used it mostly for enhancing the feel of the song.