Its about protecting the embassies from bomb blasts, protecting people of importance there, and the interests. These interests are wide ranging-and include a lot. India is spending a lot of money for Afghan redevelopment-and it is all civilian. They are training the Afghan police, building their strategic highway which links them to Iran and frees them from the Pakistani stranglehold, in rail projects, building their Parliament among a hundred other things.
Indians are in Afghanistan and working on these projects because the GoI assures them their safety in Afghanistan. For that, considering the number of Indians and physical projects and simply the size and scale of the projects India is involved in, you need a sizeable force to protect them.