In what situations should I use pan or zoom?
I find it useless pretty much. :/
It can help with creating fancy transitions probably, but besides that?

I use it sometimes to sync with the song, change the perspective, or to add a dramatic feel.
Small example[Project was abandoned because it got corrupted]:-

it can be the most important thing in the video. what many like to do nowadays to create as much flow as possible is to obviously work out nice flow with twixtor and whatnot, but then also add some pancrop to that. gives it a smoother feeling between clips and video overall.
sometimes it can be used to just give it a better feeling to the video, as noki said. it can be anything inbeween slow zooms during a buildup to pumps during beats.
some use it to just zoom in and create a line with keyframe, which the cam goes over and whatnot.
its really up to your creativity.

two examples of my own because self promotion.

Seems legit.