These are recorded as credits since earning revenue increases owners' equity. When a company earns revenue, a credit entry is made in the revenue account to reflect the increase in equity.
These are recorded as debits because they reduce owners' equity. Expenses like salaries, rent, or supplies decrease the company's assets or increase its liabilities, so debit entries are made to account for these reductions.
This framework is based on the debit and credit rules that govern changes in owners' equity.