Rag, MrFuji is explaining the maths accounting for expected hit rates - they need to be considered when dealing with things that affect your chance to hit, after all. For example, a hit with an average damage of 20, and a 50% expected hit rate has an expected average damage of 10; that is, over 100 'swings', you would expect to do roughly 1000 damage total, by the averages.
It's a well established knowledge that +/- 5 is mathematically equal to have advantage/disadvantage; GWM is essentially letting you take disadvantage to add 10 to your damage - so the more odds-on to hit a creature you are to begin with, the better value GWM is to use. The feat is powerful because players can choose to use it when it's beneficial, and not use it when it isn't.
