Double-entry accounting records each transaction as equal and opposite entries — a debit in one account and a credit in another — so the books always balance. This is what makes it possible to produce a trial balance, a profit and loss statement and a balance sheet that genuinely reconcile.
Many billing apps only keep a single list of sales; true accounting software posts every voucher to a double-entry general ledger, which is essential for reliable financial statements, audits and lending.