Dissecting metaphors: To beat a dead horse

In coming to terms with metaphors and my unreasoning dislike of them, I would like to get to know them better. Some metaphors I know the root of, and such interesting roots they have. Other metaphors make my ears bleed to hear them.

To beat a dead horse:

I understand the meaning of belaboring a point beyond which it is useful or advantageous to do so, but I don't know why this particular term is in use. Personally, I prefer to describe someone as belaboring a point beyond which it is useful to do so, but that's just me. I used to have an employee from Nepal. He would bring me copies of newspapers or emails and ask me what certain phrases meant and "beating a dead horse" was one of them. He envisioned folks standing over the bloody carcass of a horse and beating it with clubs. Understandably, he couldn't comprehend why such behaviour seemed to be so prevalent nor why some people needed to be told not to do it.

According to wikipedia, it was a description applied to Parliament in reference to that body's apathy towards a Reform Bill.

Also from wikipedia is the term used as slang for wages paid to men in advance of the work being performed.

I understand the use of the metaphors in these contexts, but I still don't fully comprehend the origins.