The Indus was mostly used for trading and irrigration. Without the river, no trading could go in or come out. The crops needed water and fertile soil. The river had so much silt that it was cocoa colored.
The Indus River is mostly in Pakistan. It originates in Tibet and flows through the state of Jammu and Kashmar before crossing into Pakastan.
The fight between India and Pakistan over the ownership of Kashmar is based partly on the fact that which ever nation controls Kashmar also controls the Indus.
Now India knew that Pakistan's army was much stronger than theirs. So by using the waters for irrigration, India could deprive Pakastan of the fertile farmland that they needed.
The two countries finally agreed in 1960 to work together on an Indus irrigration project for their mutual benifit.