I was wondering about this too. I don't really want to keep prisoners but I still wouldn't mind knowing how it works. I was kind of hopeing it would become something like. You can build a prison and keep 3 or 4 prisoners and get ingredients based on there drops you normally find when you beat them in battle. Or you can build a room have one of them live with you and they help gather ingredients near the house while your away. That way one gives you an unlimited supply of multiple ingredients and the other nicer option gives you a few random ingredients a day.
after you build a prison you can start to learn things that can help catch some things. for example you can learn to catch reptiles from the witch in the swamp she will sell you what you need but you can later learn in your self. they want you to use more then one to have a better chance but I saves mid battle and keeps reloading until I catch them. the reptile one also works on rapters so you can get an awesome mount the only thing I cant figure out is how to put the mounts back in your stable to let them rest. they seem to be stuck following me around.