It's usually scientists researching the jungle. I was a bit obsessive on the topic of wild animals as a child and the camera is a way to catch rare animals on footage to monitor endangered animals although sometimes they witness an animal dying but the scientists camps are usually miles away and there's really nothing they can do about it. They put it in nature documentaries to show the behavior of wild animals for people that want to see the genuine nature.
TL;DR: They do it for scientific reasons