To make the gore look super-duper realistic so you would feel bad about killing people.
The misunderstanding came after an article was misquoted saying that employees watched made to watch videos featuring human mutilation while developing the game.
www.ginx.tv
Note that despite Naughty Dog employees trying to tamper the rumors back then by saying they weren't
forced to watch it, they still confirmed that yes: this was a thing employees were supposed to do to get a better understanding of the gore when working on the game. So technically nobody was
forced to watch them, but it feels like it was very much a
"you better watch this snuff shit or else your job might be on the line because you won't know what the fuck you're supposed to be doing for my opus" kinda deal for the employees.
But that it's fine cuz NetherRealms employees had to do that as well. Despite the fact that the core gimmick of Mortal Kombat since its inception was to be as gory as possible whereas The Last of Us...wasn't ever that, and literally nobody ever asked for it to
be like that.