3 ideas
Most of the variation in a diversified portfolio's returns over time is explained by how it is split among asset classes, not by which individual securities or funds are picked within them. So the investor's scarce effort belongs on setting and holding the mix, while cheap index funds handle the job of filling each slot.
Combining assets whose returns do not move in lockstep lowers overall portfolio volatility below the weighted average of the parts. Because smoother returns compound more efficiently, adding a volatile but weakly correlated asset such as REITs or commodities can raise risk-adjusted return. Asset classes are judged by what they do to the whole portfolio, not by their standalone risk.
Set a target allocation and periodically sell whatever has drifted above its weight and buy whatever has fallen below it. This keeps the portfolio's risk level where it was chosen and mechanically enforces buying low and selling high. It replaces market timing with a rule that works regardless of forecasts.