
Early picture of Punch during his integration process in the cage
The cage is used for cases like Punch's where they reintroduce a macaque back to its troop. This allows them and the troop to interact with each other safely and hopefully, begin to see them as one of their own. It also helps them get used to the noises being made and behaviors. This was originally implemented for Otome's hand rearing; the area behind it opens up into a separate indoor area that they'll put the monkey for a period of time. I think this was Punch at ~3 months. There's pictures of some of the other hand-reared macaques in there as well, and they've put a macaque in the cage with them before if it's shown friendliness to see how it does with direct contact.
I've seen videos of Punch in it as well, but can't find them, and I remember them being very zoomed out so you could only really see his plushy.