Butter i great, but shortening and butter, while in some cases are interchangeable, in many recipes are NOT-
Certain cookies and pastries/crusts for example, need shortening, to turn out right- Butter DOES have liquid in it, shortneing does not,which is why,i fyou are using it when melting cocolate, butter will make it sieze, shorteningwill not.
To see the difference each makes, try your favorite chocolate chip cookie recipe, make one with butter and one with shortening (or even half shortening) and observe how the texture differs..