The main thing is more diversification. The US, being the largest economy, is a sizeable chunk of global funds anyway. If the US economy encounters trouble you'd be entirely at its mercy with no diversification if you were all in on domestic shares. Personally I think you'd have to be completely mad to hold anywhere near 50% domestic only equities, unless you're very close to retirement and you need to reduce currency risk.

Currency risk can be a double edged sword for younger investors though. If the dollar value drops, then your global investments are worth 'more dollars' so you get a considerable returns boost on your existing global investments. A low dollar also makes US goods more attractive to exporters which tends to boost the main contributors to the US indices, so domestic indices get a bit of a boost as well.

If the dollar rises in value then you get to buy more global shares for the same dollar value, so you effectively get good value for money. However your global shares are worth fewer dollars, which is why you start withdrawing back into your domestic currency as retirement approaches.