You ever played "Whack A Mole" at the local arcade? When you whack one mole it causes others to surface? That's integration. When you change one variable - such as increasing scope - it causes a change in other variables, such as schedule or cost.
The point of Integration is to be heads-up to all the consequences of a change on one area.
If you listed that nine knowledge areas in PMBOK, printed them out in a really small wallet-sized (or purse-sized) card, you'd have your crib-notes about what other areas to think about for a given change in one area. Try it. If something changed the schedule, does it change the human resources mix? ...the identified risks? ...the costs? ...the communications needs?