You cannot do edits (well you can, but you'd be using parameter automation for them, or you'd have to edit multiple tracks instead of just one or few master tracks), if the project hasn't been bounced to a master track or at least to much less tracks than what the mix has. Some claim that there's also a psychological effect of getting rid of the multitrack so you'd focusing only on adjustments that can be done to the master track, instead of fiddling around with the multitrack and its "endless possibilities for fixes/adjustments".

If the "mastering" is just slapping some plug-ins on a channel strip, then in that case it wouldn't matter.