For a diy, the fiberglass will show through if not completely covered or sanded too much. Paper is much easier to handle in that respect, but more difficult to apply. You have to make sure the tape is 100% embedded in wet compound to avoid bubbles and it takes more work to apply compound to the wall for paper. Fiberglass will stick to the wall and then apply the compound.
Sometimes you run into corners that are not tight to each other and you want to tape it now. So, you fill the void with mud apply the tape and mud that. Fiberglass tends to get pushed into the void, leaving a crooked corner. Paper is folded and makes the corner for you.