Because benzo's have a sedative effect which (some) people might find enjoyable because it calms you down. They work on the reward system in the brain which can cause some euphoric effects. Some sleeping pills might induce mild to even very heavy hallucinations. With Zolpidem (aka ambien) this is the case, especially when combined with alcohol.
But seriously, you shouldn't use them recreational. Those pills are just pure garbage, cannabis is better. Can't overdose on it and the side effects are less worse.
