It's easier for Apple to work with a company they've already developed to the standards they demand than it is to start from scratch.
In the short run, this will mean their supplier is responsible for setting up the production facility (which that supplier has experience with) and training a new workforce to operate and inspect the product (which the supplier has experience with) to meet Apple's specifications without compromise (which that supplier has already been beaten into submission on).
In the medium term, this moves the factories outside the territorial control of China, meets the demands of governments and consumers demanding a reduced reliance on China, and reduces the flow of investment into China but does it in a way that doesn't make their current suppliers feel jilted and less committed to current production and kind of boils the frog with the Chinese government.
In the long term the Indian people will gain expertise in these production techniques and then become available to start or staff new, Indian, manufacturers-- eventually becoming familiar enough to be able to advance the state of the art through their own R&D.
Excellent, thank you.