Some people like to render (bounce down) any Virtual Instrument tracks as audio files before they start mixing—a lot of Pro Tools users used to do that as a matter of course; I assume that's what's meant by "sending the pre mix song/track out of GB and then bringing it in again". While doing that does reduce the overall CPU hit while mixing [especially when there are a lot of Virtual Instrument tracks utilizing heavy-CPU instruments], and makes it easier to share the individual tracks if you wanted to send them to someone else to mix in a bigger [different] DAW, nowadays I don't think there's any significant advantage to doing it [I don't do it in Logic], especially to mix in GB, unless your arrangement is very large and you're running into CPU issues.
