India was referred to as a jewel in the British empire, but why? India itself was pretty poor during the period I think. Did they pay a lot of taxes or contribute lots of soldiers to the army or was it something else?
I am in no means an expert, but I have some basic knowledge that might be just enough to answer your question. Basically, India was full of resources, especially spieces that were very expensive. There were giant Trade Companies emerging just from the trade of Indian goods. It also allowed people to get rich easly. For example, some clerk that barely made any money in London/England could hop on a ship and live a way better life in India, because the pay was way better. So to sum it up, money was the reason that India was so important. I hope someone with more knowledge on the topic could provide some further details.
Can I add to the question: Did tea have anything to do with it? Or were the British just interested in the spices that India grew?
The importance of India has to be seen in the context of the Industrial Revolution. India was full of agricultural resources that were immensely valuable to industrial Britain. Cotton is an excellent example. The British destroyed the Indian indigenous cotton industry using tariffs and monopolies and then imported cheap cotton from India to Britain to the mills of Lancashire. Again cheap machine made textiles were exported to India to be sold to the populace.
Apart from the basic economic exploitation there were numerous Imperial sources of revenue like the Home Charges which were a major drain on Indian economy.
Militarily the British Indian Army fought in Mesopotamia, Africa and Europe in the two world wars, also in Burma. I don't have the statistics to hand, but India made an immense contribution to Britain's war economy in the first world war.