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

"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!

So is each tube individually responsible for incrementally amplifying signal?? I believe all eight tubes would be on the same circuit.

Just like a Christmas tree lighting system?

There's a reason why the Brits refer to vacuum tubes as valves....it's a bit more visually comprehensible.  The smaller signal input to the grid modulates the current flow from cathode to plate and through the output transformer.  It's akin to mechanical leverage where a small action can be magnified.  If you want a thorough understanding of how tubes work in various circuits, pick up a copy of Radiotron Designer's Handbook.  It'll keep you busy for a while.