Himilayas serve as a natural border between India, China and a couple of other countries. It's not a surprising fact given that there are countless situation of using natural obstacles as borders for countries all around the world.
Why do we do that? Well, it's easier to know where a border goes if we have some kind of a natural change in terrain to mark it. Otherwise nations would need to build extremely long fences in between of countries for example. Alternatively to that, we're using rivers and mountains to mark the borders for ourselves.
We can see similar situations with:
Ural Mountains :: they are a natural border between Europe and Asia
Tatra Mountains :: they are a natural border between Poland and Slovakia