You don’t need a unified theory of why animals have binocular vision.

The biggest threat to a primate isn’t always felines, it’s often other primates. Judging distance to your target is a very beneficial trait for survival if you need to fight them. Which means if you need to survive other primates, it is beneficial to be able to judge distance to other primates. Even if you’re not a predator.

Gorillas might be herbivores. But they are excellent at fighting eachother. And a dead gorilla can’t reproduce.