Sleeping normal mobs are crazily good. After level 100+ you will get elite mobs where you can’t one shot them, and require a full duration of dps skill such as Hail and Frost cloud to actually kill them.
Also, there are monsters that can few shot you even with high CON at later game
You can just sleep them and kill them without any worry of them retaliating.
Edit: One thing I must remind all the first time player here. Don’t assume things based on what you see in first 4 rank. The game completely change after rank 5.