An employer may not want to hire someone to be responsible for company money, when they are not responsible with their own money.

A poor credit file can show you are not organized or responsible.

Actually using much available credit is an indicator of someone in financial hardship who may be more likely to commit fraud or theft.

If you are in mountains of debt you would be more vulnerable to embezzlement and bribery. Generally only relevant for security and financial industries

It may be included as part of a background check. If you need to apply for a security clearance then any debt you have is relevant to that process.