Yes I saw the lesson, it is as Spikey says, it’s a matter of the transition you are trying to achieve, if you need a smooth one or a sharp one, it depends on your goal, so Kent uses the masks knowing what he is after. The brushes are tools that can accomplish specific tasks, but that said, is not an exact science. When you’re sculpting you’re freely flowing, like playing with digital clay, unlike when you’re box modeling where you are working inside stricter parameters.
So in other words, try not to think about it as if you have to be specifically mindful on where to use each mask or that you made a mistake if you didn’t, get comfortable at getting the feel on what tool you think adapts better to your goal. The tools are at your disposal. It’s just as easy as a chef deciding the dish needs more salt because the food feels flat, even though the recipe shows an exact amount to follow, he adjusts the amount on the fly following his perception on the matter.