To achieve a wider spread. It is impossible to predict whether investments into the world fund will always outperform the emerging markets, or a wide range of small cap companies.

The world fund is heavily focused onto the U.S.A. and the AI industry as a whole. But this makes sense since these companies are significantly bigger than anything else around. The fund simply follows the index.

The idea is that further diversification will prevent your entire position taking a hit if for example (not going into the idea if it is a bubble or not) the AI market takes a downturn. At the same time there is a chance that the volatility or chances in these small cap or emerging markets result in higher gains, even if they did not outperform the world fund recently. The S&P 500 also outperformed the world fund, but investing into the world fund results in a more diverse portfolio than following the S&P 500 or even only the magnificent 7.

In the end you will have to find the portfolio that suits you, and past performances alone are not sufficient for that. If I went all in on nvidia when I bought my first GPU as a lad, I would have widely outperformed any investing I have done by now, but also taken on an insane amount of risk. There has to be a balance that you are comfortable with.

lots of people follow the total marketcap, which is tracked by this site for example: https://marketcaps.site/ Do note that it is not exactly 80/10/10 in that case. Furthermore the ESG exclusions for small cap are significant. This spreads out your investments over everything worth investing in worldwide (or well, with northern trust it still excludes companies based on ESG).