I'll read it for you guys :
"The term ‘bus’ strictly applies to any mix bus, but often it is used to describe an audio sub‑group, which is used to gather and combine a number of related sources to allow overall control via the bus or sub-group fader.
Its output is often referred to as a ‘stem’ and typically forms a key part within the final mix. For example, you might ‘bus’ all the drum mics together to allow overall control by one (or two) faders, instead of having to juggle and balance six or more individual drum channel faders.
en.wikipedia.org
Although a fair amount can be accomplished during a stereo mastering session, having the stems of a project offers possibilities that simply are not available during a stereo mastering session.
www.sageaudio.com