Yes they probably are the most confusing ones, if you don't know where to use them

Masks are what I created them for, so the simplest example would be to take a landscape picture(preferably with a blue skye) and go to quick mask, then take the gradient tool(black to transparent) and just make a gradient from the top to let's say middle, then make some more gradients from every other direction, but start them lets say 50% outside the picture(so the result is not so dark), and the end result is a mask that masks the most edges of the picture and less on the middle... Now here's where the 4 actions come in, now still in the mask you can use the very first action, and it smoothens and gives the mask a little noise(to avoid banding) and finally inverts the mask. After this just leave the quick mask and adjust the levels(or curves, or brightness) to get deeper skies and more contrast in the edges...

The next three actions do just one of the things that the first one is doing... for example you can use the air brush in the quick mask to mask the areas you would normally dodgeburn, and then it's good to give maybe one 50pix blur and one 1% noise to the mask after the airbrush, or else you might get some banding, specially in 8bit images, after those invert and leave the mask, and adjust curves or what ever, and the end result is better the dodge burning, as you get to choose how strong of and effect you want. Of course use a normal mask for even better adjustability, but I usually use just the quick mask, as real men don't make mistakes

So basically the 4 first actions are there to make sure that there's no banding after this kind of masking, and just to make the process lot more quicker...

Also note that the Deep Sky action is basically doing what I described, so just have a look at the history there, and it might explain what I mean, though you need to have 23 history states to see all steps (press Ctrl + K to change history states)

Enjoy...