The easiest way to understand is to picture if clearances didn't exist. If you had a requirement for certain material, someone would have to decide if you were

The easiest way to understand is to picture if clearances didn't exist. If you had a requirement for certain material, someone would have to decide if you were safe, perhaps do background checks, get someone to vouch for you, etc. Clearances just standardize that, so you can trust that it's been performed within X years. You still don't share material without a requirement, you've just outsourced the other half.