There is a widely quoted statement, attributed to Disraeli, to the effect that India was the "jewel in the crown" of the British Empire. In other words, India's role was a major one -- it was the most important possession that the British had in their empire.
After the colonies that are now the United States were lost in the 1780s, India became the most important possession Britain had. It was an area that was rich in resources. It had many things that Britain wanted (including things like textiles, tea and cotton) and it had many people who could be used, for example, as soldiers. In addition, it occupied an important strategic position in the world. It allowed the British to dominate all of South and Southeast Asia.