The SR in its twitter page has given a good explanation & background for the same.The fifth and sixth lines are needed as movement of trains in the Moore Market Complex-Basin Bridge corridor is already constrained as both EMU trains as well as long distance express trains have to share the existing track.
The fifth and sixth lines will provide a separate corridor for EMU trains towards Arakkonam to ensure uninterrupted movement of suburban trains thereby reducing commuting time for passengers besides enhancing the sectional capacity.