The fundamental idea is that if you store your money under the mattress then this is equivalent of investing in the most diversified index fund out there, you invest in EVERY business and CEO out there who operates in the USD economy.

This generally is a bad idea because the Federal Reserve produces more money which results in reduced returns, BUT, this effect is really visible in fixed money supply economies such as Bitcoin. By hodling bitcoin you invest in every bitcoin business out there (including the scams) because by not competing with other investors for the same resources, you allow them to operate at a bigger scale.