Secondary Tracking segment provides better audit and analysis capabilities. You now have more visibility into the detailed components of Retained Earnings, CTA, and unrealized Gains and Losses. Instead of tracking these accounts by a balancing segment alone, you can track them by balancing and another segment of your choice, such as Department, Line of Business, or Cost Center.
It also provides better control and consistency of similar transactions because this option is set at the ledger level instaed of through profile option. By being able to nominate any segment other than your primary balancing segment or natural account segment to act as your secondary segment, you have greater flexibility in tracking accounts by pairs of accounts.