It comes down to two factors, taste and convenience. Many believe that freshly-grated or shredded cheese tastes better than pre-shredded, because cheese that is packaged already shredded is coated in an anti-caking agent that can affect taste and melting. However, many prefer the convenience of pre-shredded cheeses. One final thing to consider is price--you do tend to pay more for convenience, so you might get more bang for your buck if you purchase parmesan in block or wedge form, and grate it yourself.