How did slavery benefit some people?

It depends on what period of slavery is being referred to. In pre Civil War America, slavery allowed plantation owners to benefit from inventions, such as the cotton gin. It also greatly reduced the price of products such as cotton, coffee, and sugar, making them available to the general public, that could not previously afford them. However, the benefits in no way justified the abuses that were carried out on other humans during this time.

Slavery, and the drawbacks and benefits, is very different from culture to culture, and throughout time. Some cultures, especially early Greek and Roman, had specific laws about slave treatment. This obviously did not benefit all slaves, but did offer a measure of protection to some. During these times, slaves were used for many more types of tasks as well. Knowing what historical (or modern) setting you were referring to would allow a more specific answer. The common theme throughout is the benefits almost always came out on the end of the owner, and not the slave.