Why do Contra Accounts exist in accounts and bookkeeping?

Contra accounts are simply negative offsets to other accounts. As Allowance for Doubtful Accounts, which reduces the net value of Accounts Receivable. Or Reserve for Inventory Adjustments, Accumulated Depreciation.

A contra account is one which is offset against another account. So for example there are contra expense accounts such as purchase returns, contra revenue accounts such as sales returns and contra asset accounts such as accumulated depreciation.

If you could simply rub out your mistakes, there'd be no audit trail. A contra account allows corrections to be audited.