Benefits to home country while pursuing Masters in pure Mathematics abroad
Studying pure math seems to imply that you want to teach eventually, perhaps after pursuing a doctorate. If that is the case, and you intend to enter academia in India you have the basis for saying quite a lot. 

On the other hand, if you intend to enter industry in some capacity then you can write about the benefits of the economic activity. Even if you won't be applying the specific topics you learn, your mathematical sophistication should be improved by the study, making you a better candidate either for academia or industry. 

It only seems problematic to me if you don't intend to return at all, or to leave again permanently.