What are the different pencil/pen holding techniques for drawing? Which should be followed in which case?
There is a previous discussion here: What exercises can I do to improve the line quality of my drawings?
Holding a pencil is mostly up to the artist's preference.
Discussed in the link above, if you want to have shorter strokes or finer detail, you would want to use more of the fingertips. If you want a more fluid motion in a line/drawing, you would want to use more of your elbow.
Holding the pencil closer to the tip would also help more for finer detail, while holding it towards the middle of the pencil, would be for longer strokes.
Holding a pencil or pen in my own experience, the hand/fingers can start to get stiff/or arthritic over time. Finding myself holding the pencil differently (than we were taught in school) is for comfort, more than a specific effect.