Are you familiar with voltage dividers?So what does these Meg ohm resistors do? they just reduce the feedback voltage or current from other parallel branches or parallel paths? but why reduce it?
The Meg ohm resistors stablize the feedback voltage from drifting out of tolerance?
The larger resistor in a voltage divider will drop the most voltage across it.
As you further increase that resistance, the voltage drop further increases.
No matter how much you increase that resistor, it will never drop 100% of the voltage. You can get very close though.
Once you have a reasonably large resistor in place, as you change the smaller resistor, you will realize that the voltage across the larger one hardly changes unless you change the smaller one by a large amount.
By itself, it will only limit the amount of current through part of the circuit.
When used in conjunction with other components, it can be used to help stabilize certain portions of the circuit... This is due to the fact that when using a large value resistor, the rest of the circuit can change it's current or resistance quite easily across a very wide range of values, and the voltage across the MegΩ resistor will stay stable.
It can be used in a limited sense to 'isolate' portions of circuits from each other, but this term worries me, as they are not isolated with a large value resistor, they just have a greater resistance between portions.