Can anyone explain what a power tube does inside an amplifier, eg kt88.

Read the beginning of this:  

https://worldradiohistory.com/BOOKSHELF-ARH/Technology/RCA-Books/RCA-Receiving-Tube-Manual-1975-RC-30.pdf

Oh my God, the document above is 760 pages long.

It looks pretty damn awesome and it has pictures.  I wonder if it was ever translated into another language.  Fortunately I only can read English and how lucky I am this document is in English. I failed German when I was in high school. What an awful experience that was. 

Why is it necessary to have eight Power tubes on a mono block amp? Assuming its 300 watts, does this mean each tube is incrementally amplifying the total watt availability? Why not just do a few tubes, or would that means less watts available?

That's a lot of vacuum space for electrons to flow through going through so many tubes so quickly. I recall seeing one amplifier with a single tube about eight times the size of a kt88.

Takes space for all those sweaty little charged particles to do their thang...😏

SS stuff makes the work easier....and less expensive...

Well, originally...anyway...;)

"Why is it necessary to have eight Power tubes on a mono block amp?"

A KT88 tube is rated at about 35 watts per tube. That is why you need 8 tubes to produce 300 watts!