In my opinion, the most likely reason that a farmer would release wasps into his field would be if he meant to use them as a form of biological pest control. In other words, the farmer might be using wasps as a form of pesticide so that he does not have to use chemical pesticides.
This can be done if the farmer has problems (or wants to prevent problems) with some sort of pests that the wasps can prey upon. He will then release the wasps and they will kill the pests. That way, he does not have to use chemicals to kill the pests.
The link I have provided shows one example of using wasps in this way.
This may be a step taken by the farmer to control some oter kind of pests whos predator is the wasp. The method is called biological control method (the releasing a predator of the pest). This generally has may advantages (but if the right kind of biological control organism is directed) such as: no need to release the organism over and over again as they reproduce naturally and it is not harmful like the pesticides.