This feedback locks the output of the VCO to the exact frequency of the bus clock. You've got a stable, temperature independent clock generator on-board the CPU that's operating at whatever speed you want. Before you actually use the chip, you write to these control registers to configure the chip so it will operate the way you want it to. You can think of these control registers as command line arguments. Whenever you invoke a utility like gzip, you put a string of switches after the program name to specify how you want the program to behave.
Control registers are the exact same thing.
Whenever you fire up the chip and initialize it, you set certain bits in its control registers to tell the chip how you want it to behave. You manipulate these bits in the control register by putting the right voltages on the input pins at the right time. In most ICs, and especially in most CPUs, there aren't any pins to spare. Wasting pins like this is a bad move. If you're smart, you'll get around this by making some pins do double duty.