The benefits of having several different asset classes in your portfolio are well-known. There are dozens of increasingly exotic asset classes available to invest in.
Investors consciously and subconsciously project the recent past into the future, despite the well-known fact that past performance is no indication of future returns.
Historical correlations are relatively easy to look up. Correlations vary over time, but you’re looking for an asset class that is non-correlated with your current asset classes as much as possible.