The main reasons for building an investment portfolio with multiple assets are to diversify risk and stabilize returns, which is known as asset diversification. Here are the main reasons:

1. Diversification of Risk

Different assets have different characteristics and risk factors.

Even if a particular asset or market (such as the stock market) declines, other assets (such as bonds, commodities, etc.) may offset some of the losses.

For example, when stocks decline, bonds, as safe-haven assets, usually increase relatively.

2. Stability of Returns

If one relies solely on a single asset, the portfolio may experience significant fluctuations due to market volatility.

Including multiple assets can compensate for the losses of one asset with the gains of another, thereby making overall returns more stable.