PE parts are used when styrene parts would be too thick and therefore out of scale, such as mesh engine covers, ship railing, ect. Also they are used where the plastic part lacks the detail which can be attained by using PE.
It is pretty simple stuff to apply in 90% of the instances you use it. Usually, remove it from its "sprue" with a sharp pair of snips, file the edge where you removed it from the sprue and glue to your model with CA (super) glue. For the other 10% bending and tweaking of the PE is necessary. I will usually place the PE under a # 11 blade and bend it up along the edge to get the desried shape. For curved shapes, I will roll the PE in my fingers. Even compound curves can be made this way, it just takes a little practice.
Good Luck
N.