Cos every transaction requires debit and credit. If asset is bought, we debit asset with the amount paid and credit capital with the amount paid (if paid by cash, cheque etc). So at the end all must be equal.
Trial Balance Credit and Debit Must Balance by: Stephen Alabi
The trial balance must be balanced because from the accounting equation asset=liability+capital so that means the debit of trial balance which is used for recording the values of asset and expenses must be equivalence to that of credit side used for recording liability, equity/capital and income...