Nothing in particular. It was just a feeder line constructed mainly to cater to passenger traffic. There was a ferry over the Damodar river which this line connected to. Because of the high population density, the Bengal province had several such light railway systems to act as feeders to the main lines for passenger traffic. You can search for McLeod's light railway, Martin's light railway and many others to get a good idea of the number of light railway systems that were operational at that time.
Also, 'British always had a reason behind investment' is not really a correct statement. A lot of the MG and NG routes (Even many BG) were constructed by the British for...
more... famine relief - for easily supplying large amounts of food and resources to famine-hit areas. They ensured good returns on their investment through good planning and execution, partly helped by the lack of severe political interference we have now. The best example of this is the financial status of IR now. These days IR loses money on Sleeper and unreserved passengers, despite the huge amount of traffic. But back in the day, 3rd class traffic, even though it had the least fares, made up the bulk of the profits for those railways while the fancy 1st and 2nd class traffic only had marginal profits.