Why?  Stability and convenience.   Shooting with your DSLR mounted onto a copystand is great for shots where You have limited space for a tripod.  Putting a tripod on a desk is tricky.   Copystands are meant to be put on a desk. Face-on shooting is needed.  This is needed when shooting paintings and scanning documents/slides.  Ensuring that sensor plane is parallel to the copystand base, and thus any object placed flat on it, is easy.  Just print a calibration grid (any set of grid lines will