I am pretty much a non-expert. I accept that power MOSFETs have a lot of gate capacitance because the data sheet says so. The manufacturers wouldn't put so much capacitance in by adding capacitors just for fun, so it must be the physics of the MOSFET that causes the capacitance. If you are asking about why the physics dictate the capacitance, you are ahead of me.
as for the effect, the gate capacitance demands quite a lot of current, fast, to get the MOSFET to act as a high speed switch. It is good to be a high speed switch because the time between "on" and "off" is when power waste happens in the MOSFET. Charge the capacitor really fast, and the MOSFET changes from from (zero current and zero power) to (some current with some voltage causing some power), to (lots of voltage, no current, no power), quickly.
This is the viewpoint of me and my mule. If it helps, fine. If it doesn't, somebody else will be along and try another explanation in a little while.