The most important thing about sliders that I have figured just in the past 2 years is this. When you bring the ball up behind your body, you need to make sure that the arm does not cross behind your body. If it does, the arm won't be up to the power position on time and you will have a loopy Frisbee slider. The grip is also a key, you want to make sure that the last finger to touch the ball (in the sliders case it will be the index finger) is on a seam to make sure you get that last release with the proper spin. The slider is the breaking pitch that is thrown the most like a fastball. When I'm doing clinics these days, and teaching breaking pitches (sliders, curve balls) I remind the pitchers to think 'fastball' until the very end right when you are leaving the power position, then slider, which is when you are to have fastball arm speed when you release the breaking pitch. These are all mechanics that require precise timing, which is brought on by doing the action over and over and over, lots of repetitions.
All in all, the slider can be a go to pitch, a strike out pitch, a setup pitch. It all depends on how it moves, and once you figure that out, then how you move it.