Because there was a market for it. The war disrupted food supply in Europe, so the Greeks wouldn't have been able to gain access to grain from the Balkans. Another key source of grain was Egypt, but Egyptian grain went to military requisitioning because it was home to Britain's largest military base outside the UK, so they didn't have any extra to export. So, supplies were brought in from India to fill the shortfall.

The British did have a bit of a tendency not to notice they were taking too much -- they did this in Egypt during WWI, so were probably trying to avoid that mistake and wound up causing famine in India instead.