FX Busses and CPU?

What is it about putting FX on separate busses and using sends from tracks that saves CPU capacity?

Let's say I have an FX buss for each effect, for each instrument:
Lead Vocal (compression, chorus, reverb)
Acoustic Guitar (compression, chorus, reverb)
Backing Vocals (compression, chorus, reverb)

For this simple project I have nine busses for FX.

How is it that I am saving CPU capacity if I have these on a stereo buss rather than on a single track? I am running the same number of FX.

Thanks in advance,

Tom

busses are mainly for workflow. if you've got your string section tracks balanced the way you like, you can send them to a bus for group FX and change their volume with one fader instead of four - and you can adjust the balance without ungrouping them all (for those of you about to say, "but you can group the faders!").

you can also save CPU cycles as marah said - do you really want an instance of VC64 for each track of your doubled vocals and BGVs? probably not.

some FX, specifically reverb and delay, work best on a bus of their own. i've also used sends to a compression bus and a VC64 instance for various reasons. with OHs i've added a send to a compressor so i could add in just a bit of "drum destroyer" to bring out the cymbals without slamming them entirely. same thing with some VC64 presets that are way too much when in-line.

finally (although i'm sure there are other reasons), with some effects like compression you can get the multiple instruments to "play" off each other in ways not possible by single-tracking everything to a master bus.