Most purchases are not paid for with cash, but instead are invoiced and then a period of time (credit) is allowed before payment is due.
In order to record these transactions we must split it into two parts:
A sale will lead to a Cr entry to the sales account. The Dr entry will now go to the Receivables Control account upon invoice.
A purchase or expense will lead to a Dr entry to the relevant account. The Cr entry will now go to the Payables Control account.
Until payment there will be a receivable (cash owed to us) or creditor (we owe cash). The payment or receipt will clear this.
Upon receipt, we will Dr Cash account (as usual), but this time the Cr entry will be to the Receivable Control account (removes debtor).
Upon payment, we will Cr Cash (as usual), but now we will Dr the Payables Control account (removes the creditor).