If you want to do an operation on fooA, you don't mutate fooA. You call fooB = MyFunc(fooA) and use fooB.
The nice thing here is you can pass around pointers to fooA and never worry that anything is going to change it underneath you.
You don't need to protect private variables because your internal workings cannot be mutated. Other code can copy it but not disrupt it.
This is the bit I don't get.
Why would I do that? I will never want a fooA and a fooB. I can't see any circumstances where having a correct fooB and an incorrect fooA kicking around would be useful.
But also keep in mind that correct and incorrect is not binary. You might want to pass a fooA to another class that does not want the fooB mutation.
If you just have foo, you end up with situations where a copy should have happened but didn't and then you get unwanted changes.
You say that value2 is correct. It logically follows that value1 was incorrect. Why did you assign it then?
The names are free, you can just use a correct name every single time.
The world by definition mutates over time.
The beautiful thing about this is you can stop naming things generically, and can start naming them specifically what they are. Comprehension goes through the roof.