A Rabbit pen may not just contain Rabbits! But it's function is to provide a place where wildlife can always be found for training steadiness and working anongst distractions. Imagine retrieving a dummy with rabbits running around!
Most people dont have access to that kind of thing, but we still do what we can. Walking in an area of high rabbit density, doing stays in a wood where there are lots of Squirrles, walking through a chicken run or beside a Pheasant pen. All ways I use to the same end.
Regards, John