Sri Lanka is in economical chaos. They cannot get enough dollars to keep their imports going. So they are keen to use other currencies for their foreign trade. India can give loans in INR, which Sri Lanka can spend only in India. They will for example buy energy in India for their northern area.
For ordinary Indians nothing will change. For the upper class, there are trade possibilities.
The benefits are in there: the trade opportunities.
I'm not an expert, please correct me if I'm wrong.