The only sensible reason for keeping humans in the Matrix

There have been many discussions on if humans are imprisoned in a simulated reality such as presented in the movie The Matrix. In the Matrix, the reason for imprisoning humans is to extract energy. A reason not very plausible, since it would be much less trouble to keep, say, pigs. They could perform the same function and cause much less razzling. In the book Under the skin from 2000 Michael Faber instead keeps the humans for meet production. Now, also this is something pigs would do better.

So, what reasons are the for aliens or machines or other humans to imprison humans. Well, I can only see one: for the computing power of our brains. It is reasonable to assume that there is something unique with the human brain. Unique enough that it could be worth the trouble of creating a simulation like the Matrix. What do you think? Any other reason?