You can use them to apply the same effect to different parts of your song. It is basicly CPU saving.

Say you have 4 synth parts you want a big bad reverb on.. so instead of using 4 different reverbs who needs a lot of cpu you just add the reverb to one of the busses instead and you send the 4 synth parts to the bus you put the reverb on. Do you get it?

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enLounge - Secret Beach