Why do accountants say a favorable bank balance must be shown as a DEBIT in the books of the business?

In the books of the business, a favorable bank balance is an asset.

An asset account has a DEBIT balance brought down.

If you deposit money into the bank account, you will debit the bank account, so the balance increases.

If you withdraw money from the bank account, you will credit the bank account, so the balance decreases.