Set off of debtor and creditor

While working on accounts finalisation I found that our client company has both purchase and sale transactions in a number of instances with the same party. Therefore the same party is both Debtor and Creditor for the Company. In all the cases, both the parties ie. our client company and the external third party are maintaining two separate ledgers namely 1. Debtors Ledger and 2. Creditors Ledger. However for the year, my client company is not passing a set-off entry between the Debtors and Creditors ledger and keeping both the ledger balances as it is. My question is what is the point in maintaining 2 separate ledger balances with the same party as this ultimately will be adding weight to the Balance Sheet and make it unnecessarily heavy.