Mixing is the second to last step in finishing a project (mastering is the last). I mix my own tracks and well as tracks from other recording studios. During the mixing process, we can correct minor mistakes, add EQ, compression, pitch correction, effects such as reverb or delay, as well as writing in dynamics automation.
Since mixing in Pro Tools is total recall, most artists today get me to mix the songs where they sound good to me, and then we get together to make changes and tweak to satisfaction. In the old days of analog, even with board automation, mixing was a whole different animal. If you didn't like the mix a few days later, you had to spend the time getting the mix back to where it was originally to make changes. Some artists still prefer to be here for the whole process. It doesn't matter to me. However, a lot of what I do in mixing is cleaning up tracks, adding basic EQ, compression, and dynamics automation that is almost always going to be done. This is the most boring part of the mixing process and I can do it much more quickly and efficiently by myself. That way, when you come in to listen, our ears are fresh and everything is pretty close to where it needs to be, and it is actually fun to finish the mix!