What is the function of Boeing C-17 Globemaster III in Indian military?
The C-17 serves the same purpose in the Indian military as it does in other armed forces: it provides large-scale airlift. 

India is a large country that has long borders with unfriendly neighbors, so it has a requirement to move troops and supplies quickly to any hotspots. 


  "The Indian military needs to do three things: augment its ability to quickly lift larger numbers of troops as it views possible threats on its border with China; strengthen its presence on the Pakistani border; and fight terrorism and low-intensity warfare", said a senior Defence Ministry official. "India needs to triple its lift capacity", said the official.


India spent on the order of $5B buying these aircraft, it's unlikely they'd pay that much for aircraft they don't need.