Marcie - benzos are often used for the purposes that you mentioned. But since one of their major side effects is drowsiness/sedation, they're often used as sleeping pills as well. They are also abusable in terms of developing dependence on them with overuse (basically you can get hooked on them) and therefore, that's why they're on a urine toxicology screen.
More information than you ever wanted to know about sleeping pills ... :)
Masaki
