Diversification reduces risk. A lot of academic work has gone into showing that diversifying your investments is essential to protect against any shocks in any one market or geographical area. Even without the research data demonstrating the truth of this, intuitively it makes sense. Political upheavals are likely to be localised, as are natural disasters, and one economy could be doing well whilst another languishes. Being invested across the globe means that a downturn in one geography can be offset by good performance in another.
It is also the case with asset classes. Bonds and equities have often been negatively correlated, meaning that when equities have underperformed, bonds have been a cushion to absorb some of the equity weakness. Infrastructure investments, where growth frequently comes from increases in public sector spending, can take up the slack when economic growth in the private sector is weak.
At the company level, a company in a booming business area may still not provide good returns for investors if management is poor. It makes sense to own lots of companies across multiple business sectors to raise the likelihood of making good returns and reduce risk. It is impossible to predict what will happen in markets. A well-diversified investment portfolio means you don’t have to.