Most jobs use it to see if you are a responsible person, going by the assumption that a responsible person would never allow themselves into large debt. Also, I know for higher security jobs they use the credit score as a way of assuring trust. Someone with a good credit score and little to no debt has no reason to steal from the company because they wouldn't be desperate.
Crappy reasoning in my opinion, seeing as how one may need said job in order to pay-off debts....