India is a large economy and a small share of global market capitalisation. A portfolio built entirely from Indian listed equity is concentrated in one currency, one policy regime and one economic cycle — which is fine until those three move together in the wrong direction.
That is the whole argument for international exposure, and it is worth being precise about what it does and does not do. Diversification reduces the impact of any single market's bad decade. It does not raise expected returns, and it introduces a factor Indian-only investors never face: currency. When you hold dollar assets, your return depends on the asset and on where the rupee goes, and those two can pull in opposite directions.
The articles below cover the case for going global, how currency actually affects returns with worked numbers, how much of a portfolio might sensibly sit abroad at different income levels, and the routes available for getting there.
Allocation guidance here is educational framing, not personal advice.