Volatility is not a stable quantity. It clusters, spikes and mean reverts, and the degree to which it does so is itself tradable — directly through vix-options and vvix, indirectly through vomma in any options book.
A high vol-of-vol environment rewards convex positions and punishes anyone who sized a short volatility book on average conditions. It is the reason a strategy backtested through a calm period fails in a nervous one even when its directional assumptions hold.
Example: XYZ 30-day implied volatility has traded between 18% and 24% for months, then jumps to 39% on a sector shock and back to 26% a week later. Nothing about XYZ's price range changed dramatically; the price of insurance did, and that is vol of vol.
Related: vvix, vomma, vega-convexity, vix-options