You can examine the Mona Lisa to study Leonardo's actual brush strokes, materials, underpainting, etc. It's hard to make an exact copy of it that can provide the same insight, though high-resolution 3-d scans (including x-ray/MRI/etc.) could help.

However, some artwork is lost to time and all we have are copies. Those copies may have lost important features from the original work, though they may provide additional insight into the artists who created the copies.

With digital works, one can usually trivially create an exact copy.