Legally they cannot use your credit score as the ONLY means for denying you a job. But they can always find another way to deny you if your credit is bad. If you are going to need a bond then they may not be able to hire you if you are not eligible for a bond. If they are checking your credit they have to tell you in the statement. The purpose of checking credit is to see if you meet obligations. For any job with secrecy the reason is to see if you are vulnerable to coercion. That mainly comes into play with jobs that need security clearances.
The background check will show addresses you lived at, and jobs that you held and any arrests and convictions. The main purpose is to see if you are honest.
In realty credit scores have become a way for everyone to make more money off of a person. You pay more in insurance, higher fees, higher interest rates etc. And since no scores are the same then what is the use? Your score can go up or down 50 points from each monitoring agency. You can apply for an FHA mortgage with a 660 but can't get many credit cards with that score.