Why must debit equal credit?

This is the basic concept behind double entry bookkeeping. If the debits don't equal the credits, something is not being accounted for. Example, you deposit $1,000 in the bank and record $800 in sales. Where did the other $200 come from? If someone paid off an A/R, you need to reduce the A/R or the account balance is incorrect. If it was a loan from the owner, you need to reflect that liability.