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

It is a valve that opens and closes in response to the input signal fed to it. Output side, (anode or plate), is connected to a high voltage power supply, that feeds the output transformer connected to your speaker through the tube. The input side, (grid), accepts the tiny input signal from your preamplifier, and turns the large power supply on and off in response to that input. The result is a much larger signal being applied to your speaker , than the tiny input signal being applied to the grid. Basically how amplification of the tiny input signal is achieved. 

Of course there are more parts to the tube that sets the amount of current bias is applied. But the above is a simple as I can attempt to explain it.

So does the plate collect the electrons until the charge is enough to push through the gate? And what prevents loss of  information? Once the tube gets warmed up, I guess it is just continous?

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.