PWHT for API 650 Nozzles?

As per API 650 requirements ,it is required to PWHT nozzles as the thickness of shell exceeds 1/2 inch and material group IV,my question:

1) What is the idea behind prohibiting PWHT on tank after erection on tank?

2) Is it required to perform PWHT only in a furnace or we can perform it with PWHT machine in site before erection?