AlChuck said:
It's so you can route groups of signals together and then treat the grouped signals as a stereo pair.
most busses, at least in mixers not specifically designed for recording studios, are mono.
so you wouldn't put all your drum tracks on buss 1, unless you wanted mono drums.
on the other hand with pretty much every mixer i've worked with though, the buttons for sending individual channels to busses are linked like they will be
treated as a stereo pair... i.e. there is a button for sending to busses 1-2, another for 3-4, etc.
then you pan the channel either left or right depending on which buss you want it sent to (1 or 2), or leave it in the centre for both equally. Then, you can use busses 1 and 2 like they were L and R in a stereo pair. they each have individual mono outputs.
also, the master stereo fader is basically just another pair of busses linked together on one fader (or on larger mixers, often split into two, giving it no distinction from busses other than a different name.) you're also usually able to route more things to the master fader, like tape-in and every aux bus and matrix. but that's the only difference.