star wars - Why are droids tortured?

Bitmask took this photo while attending the Star Wars Identities Exhibition :

I remember this scene from one of the original trilogy movies. I also remember the droid was "screaming" as it was tortured.

Why would someone torture a droid? How could it be effective? It's a Robot!

Answer

TL;DR See at the bottom of this post after both pictures.

The reason is two-fold. First of all, what works for humans also seems to work for (sentient) droids. 3PO is clearly shocked by the torturing, motivating him to do good work for Jabba (which he clearly despises). The picture I took was the script version of this shot from Return of the Jedi:

Where a droid that very likely was only designed to feel pain, is tortured by the assistant droid 8D8.

The true reason for this whole mess is a certain droid, EV-9D9, that started its existence in Cloud City. It later worked for Jabba as some sort of administrator for his droids. I cannot describe it better than wookiepedia, so here's the important excerpt:

She [EV-9D9] kept a laboratory deep in the bowels of Jabba's Palace, where she built grotesque droids from extra parts that existed only to feel pain. During her years spent there she added several upgrades to herself so that she could feel more of the droid equivalent of pleasure while watching the coolant from dismembered droids pour out of them while using her droid assistant 8D8 to burn the helpless victims.

... and here's the handsome lady:

So to summarise, the reason EV-9D9 tortures droids is because she genuinely enjoys it. And the reason for Jabba to allow and encourage this is because it works a "great" way to motivate his droid-servants.