What @gemiwing said.. being stoic means not allowing others an inkling of physical or mental pain. Apathy is entirely different.
Now, I can see a great benefit from stoicism. If no one knows you’re hurt, you can’t be taken advantage of in your weakened state and you won’t be singled out or pushed out of the group. The illusion of strength/health might be appealing to potential mates, as well.
Lots of animals practice stoicism and it’s a survival mechanism. If they act weak or sick, they might get preyed upon or cast out from their group.