The reason why bank credits your deposited money

See it from the perspective of the BANK

Checking Account: Credit UP
You depositing is a form of LIABILITY of the BANK.
The bank needs to pay you that deposited money anytime you want

The bank then increases the credit of your "checking account"

Cash Account: Debit UP
What they debit instead is the bank's own cash account.
Meaning their own money has been raised.