Hey there

Not all derivations are important but there are various derivations which you should practice so that you know how to start with questions basically numericals based on derivations like derivations of electrostatics are important and derivation of optics are also important it is because questions are moulded in such a way that just formula will not going to help you but you should know how to derive it. Ofcourse no need to write all steps in exam just to the point solution will be enough.

Good luck