In my mind the reason for this is often increasing government transparency and encouraging citizens to be more critical of the government by exposing them to the gruesome realities that most of us don't see often. I think that the hypothesis here is that violent acts or murders committed for the camera is fairly rare, which seems fairly likely based on my observations. I'm not sure though and would be interested to hear from someone with more information.
I think it works both ways. On one hand some videos definitely seem to be published with the intent of exposing the horror of war (Collateral Murder comes to mind), while on the other hand some just seem to be straight up snuff that glorifies the violence.
It is a tricky subject, there probably is not a single correct answer.
