Well, mixing is something you can't finish without vocals. Also, seeing as how I amost always start mixing by making the vocals sound right, I couldn't really do any mixing until the vocals are done.
mixing with vocal tracks allows every instrument in the mix to work with and around the vocals. Also by mixing with vocals included, you can utilize duckers to help balance the mix between vocals and other instruments that occupy the same frequency spectrum. Try doing that with vocals tracked over a 2-track mixdown and you will get the karaoke effect where everything drops when the vocals come in.