What role did India play in the British Empire?

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.