1 EV-9D9: We have been without an interpreter since our master got angry with our last protocol droid and disintegrated him.
1 C-3PO: Oh my!
2 R2-D2: Buzz-whoooo!!
2 EV-9D9: You’re a fesity little one, but you’ll soon learn some respect.
3 EV-9D9: Take him away and torture him for a while.
3 R2-D2: Wirr-wheee!!
4 C-3PO: I always thought it odd that droids have pain circuits.
4 EV-9D9: Now you know why.

Why would anyone program a droid to be able to feel pain??

It seems utterly ridiculous.

On the other hand, humans feel pain as a warning system, to prompt them to remove themselves from harm as quickly as possible. Pain is useful for humans. So you could imagine that robots equipped with the ability to at least register something more or less equivalent to pain would be better equipped to protect themselves from danger. Of course that registration could be done simply as damage detection, feeding into an appropriate response algorithm. It doesn't have to translate into a sensation that the robot finds as horribly unpleasant in the same way as a conscious being.

It'll be an interesting question for some time in the future.

Or a long time ago, in a galaxy far, far away....