FPR and Individual Throttle Bodies?

Why many companies sell FPR with their ITB's?

I see that they are selling FPR despite the fact that the kits are tailored for cars that already have FPR...

Any good reading on It from scratch till the end....

I am search of how IACV etc etc work together for a daily basis car with ITB's.

Often ITB kits will run a different pressure from the OEM setting, so it will be required, anyway.