My guess for why they're doing these experiments is that they must be trying to figure out what kind of people from the other worlds are capable of, or to be more specific, how strong their hearts are.

See the heart is made up of both darkness and light. And it's important to have both in the balance, too much of one can lead into a lotta trouble. There were many that experiment to see how powerful darkness can be, that brought about the heartless. There are many others who like to try to control the darkness too I'm afraid.