Protein Purification - (Aug/06/2006 )

I want to know after purifying protein through affinity column why one have to again purify through Superdex and Resource-Q column.

Apparently there are minor comtaminants exist after the affinity column. It could be some other proteins with his residues that minic His6 function if Ni-NTA column is used, or it could be partially degradated the same protein by protease.