I’m seconding what @TobinL and @anon61221983 said. Anybody who comes into contact with sensitive or classified information is required to go through these screenings, and the government outsources a lot of work.
- Aerospace company on DoD contracts? Likely clearance work.
- Network operations in DoD, DHS, FBI, OPM etc.? Yeah you too.
- Janitor in a facility with a [SCIF][1]? Probably.
- DoE employee or contractor dealing with atomic energy? Yep.
- DoS staffer? You might as well.
- Various members of state LEAs that work in “fusion centers”.
- All of the people that handle government payroll.
ETA: I forgot to mention the high profile and publicly favored NASA. I’m sure I’m forgetting others as well.
…and we haven’t even got to the big Intelligence agencies or actual military personnel yet.
[1]:https://en.wikipedia.org/wiki/Sensitive_Compartmented_Information_Facility