Why High Plate Voltage in 2A3 AMP?

I see, thank you, Herman.
So is there any benefit of having high B+, resulting in high plate (to ground) voltage?

Thank you, Ed and Herman.
Having graduated from building a couple of kits. I feel that I am ready to build one from scratch. Researching schematics, gathering parts, etc. Hence the question. It will be a while for me to understand your insight completely, however from this thread I have learned several useful things.