Back in the day such investigations were done for three reasons, national security, financial control and law enforcement. Then anything doing with 'kids'. Those all make sense.
When I worked for Bechtel they did one for me (on me?) so I could handle security documents, including stuff like wiring runs - including the security systems - for nuclear power plants. OK, I can see that. Likewise, anyone who is going to be handling a lot of money, or have control over a lot of money is going to have a credit check run on them. After several cases got a lot of press where schools and day care centers had hired people with some sort of sex conviction, they became required for that too.
But its become more common to run them on just about everyone these days because of 'due diligence' on the part of the company and it's insurance carriers. You would not think that like, oh, a janitor would need one, but a few years ago on the East Coast a girl was raped by a janitor in some big rock concert, who sued the company, now everyone gets one.