What is the need of Neutral and Phase Metering?
You would typically monitor the neutral as part of an anti-tampering technique.  People might bypass only the line , in which case the return current through the neutral would be different (higher) than what is on the Line. If both the L and N were by-passed then the neutral voltage might change (depending upon when the ground connection is made).

On Edit: looking at the datasheet it says " In anti-tampering mode, the power difference threshold between L line and N line can be: 1%, 2%,... 12%, 12.5%, 6.25%, 3.125% ..." on page 13.