What is the reason behind Gazwa-E-Hind?

As per my understanding, Gazwa-E-Hind is the prophecy that one day war will be brought upon the Indian subcontinent to invade these lands. Two groups will lead this war on the Indian subcontinent. But my question is, why will there be a need for this? Why bring war? What's the background behind this hadith? Why only the Indian subcontinent and not any other land mass? Please clarify.