Asset managers have different approaches, and I don't wish to suggest there is only one way to run money. There are many ways one can attempt to reduce risk, improve performance, lower drawdowns and reduce volatility.
No one knows what the top-performing asset class will be next year. Lacking this prescience, your next-best solution is to own all of the classes and rebalance regularly.
Owning a variety of asset classes means that some part of your portfolio will be doing well when the cyclical turmoil arises. A broadly diversified portfolio includes large capitalization stocks, small cap, emerging markets, fixed income, real estate and commodities.
Mutual fund managers want your money in their funds. They get paid based on assets under management.