Superficially, Sleep needles are for sleeping monsters you want to stab. It's wasteful to shoot them at high EV targets, so they're for high AC low EV targets that a ninja would otherwise have difficulty hurting in melee.
This isn't a great plan. The needles can miss, another foe can arrive and shout, it likely only works 1v1, and shooting a failure-prone needle in melee combat is a rather weak play, while shooting it at range gives the foe time to awaken while one closes.
So I've often wondered what Sleep needles are really for. And I finally figured it out. The answer is, paradoxically, "Nothing... by themselves."
The true affordance of sleep needles can't be understood without first understanding the affordance of throwing nets.
The purpose of a net is to nerf the EV of a low-HP dangerous caster, ideally one that is slowed and has blink. A caster will stay in a net longer because he can still cast in it. The net neutralizes his primary defense of EV. If he blinks out of it, the net remains intact for future use. If he's killed while inside the net, chances are good the net will survive.
Here's an interesting fact: Previously-inflicted poison damage over time does NOT awaken a sleeping target.
Thus here's an overpowered sequence:
- Net high-EV caster
- Curare (can't miss). Target slowed.
- Poison to max.
- Sleep needle
- Stab, or just break LoS and let him tick to death if it's quiet