I believe joint compound is primarily gypsum with some sort of binders and water to keep it plastic until it dries. The outer layer of sheetrock is paper, while the inside is gypsum.

Evern the best sanding job will result in seams that are a slightly different texture and consistency than the sheetrock itself. You need to use primer before painting it or else the seams will absorb the paint differently and end up making the wall look funny, unless you use multiple coats of paint.

I have home improvement books that recommend joint compound to fix cracks or flaws in plaster, because joint compound will fill small cracks and doesn't shrink while drying, unlike spackle.