The HSL color model allows the color to range from a color that is fully saturated to gray. The HSV goes all the way down to a white, which is not useful to many users of the color model. On the other hand, light can range from white to black in a HSL model (with the desired color in between), while the similar HSV color model can only range from the desired color to black. It is obvious that the HSL color model does indeed offer more freedom
Now for the bars at the bottom of the dialog. The color bars at the bottom show your color before (the top color bar) and your color after (the bottom bar). The color inside the middle bracket (which shows up when you are targeting a specific color) is the main color that you will be affecting and the colors inside the outer brackets are used to feather your selection. You can resize any of the brackets to fine tune your selection and narrow down the color that you want to affect.