MgCl2 (and EDTA) in buffers - (Apr/19/2012 )

depends on the protein you are attempting to purify. it could be to prevent binding to other proteins or aggregation. or it could be to release your protein from another to which it may be bound. it could change the conformation. the protein of interest could orient around mg. it could be to overwhelm other cations (selective binding).

and other possible reasons