I see it as a hedge against inflation. In boom times with soaring markets, there is (usually) considerable inflation. If you ride the market, you probably won't see outsize returns but you won't lose purchasing power to inflation either.

For this reason I frequently wonder whether I should invest HSA funds exclusively in medical stocks. If healthcare gets more expensive, my HSA has probably grown. If healthcare gets cheaper, my HSA might shrink, but that's fine because healthcare is cheap.