Among other things humans are fertiliser. Orcs are capturing humans as slave labor and fertiliser. What do human bodies produce/store that plants need to grow?
Nitrogen, Potassium, Phosphorus, Calcium, Magnesium, and Sulfur
We humans need these elements. Bones, neurons, muscles, and animal bodies in general need these things for structural and functional purposes. These animal bodies need to decompose in order to make these nutrients available for plants.
Plants also need carbon dioxide. No carbon dioxide, no breathing for plants. You and I (and other animals) give up carbon dioxide when we breathe. More than 95% of carbon found in plants comes from the atmosphere. As a side note: cremated bodies buried under trees don't become part of the tree. The tree isn't interest in the carbon its roots find and the cremation process burns off most other compounds. If you want to help a plant grow, just keep breathing.
The only advantage humans have over other animals is intelligence and adaptability. That is why humans were used as slaves throughout history. Animals are notoriously inflexible and untrainable except for routine proscribed tasks. Humans have the ability to adapt their performance to very diverse scenarios. This is also why robots have not yet taken over all manufacturing tasks of human labor.
Otherwise, humans would be particularly useless to Orcs. Other animals would be much more useful, less troublesome, easier to manage, having a shorter and more prolific reproductive life cycle, and being lower on the food chain, more economical to raise. We also have very little nutritional or food value, with respect to other animal food choices. That was our main evolutionary advantage - we just weren't appetizing to other predators, so they left us alone. Too skinny, not enough meat to bother with.
Really, if it is all about just fertilizer and energy sources, humans are a very poor choice for domestication. Like farmers clear fields of weeds to increase production, humans would be among the first species Orcs would want to totally eliminate and not keep around. Sort of like what the white man has done to indigenous peoples.
It is only because humans are basically narcissists that we THINK we could, or should, be useful to Orcs, and that we THINK there should be a good reason for them to keep us around.