Doing your master in the same session as the mix usually limits you from using high-quality audio software plug-ins on your master. Limiters for example (and many other plugins) are better at higher oversampling rates or higher quality settings.
A busy mix might be full of CPU once you start adding plug-ins to the mastering chain. But if you bounce that mix into a new session. You will be starting from a clean slate for CPU and you can freely use as many plugins as you will need and with as high-quality settings as you want