Well, say you retire at 40 with 400k in Trad IRA, 40k in HSA, and the rest of your money in taxable.
Then, every year of retirement, you max your HSA contribution, and then convert an amount from your Trad IRA to Roth equal to that year's standard deduction plus your HSA contribution. Then say you get an unfavorable sequence of returns that sees your balances drop drastically at the start of your retirement and stay low for several years. With the market low, your conversions drain your Trad IRA low enough that even during the market recovery, you're converting money out of it faster than it can recoup its losses. Many years later, you finally end up draining it down to zero, and all along you've been doing a max HSA contribution every year in order to speed up your IRA conversions. Now you've got a fat 6-figure HSA that keeps growing with market returns, and until 65 your standard deductible is "wasted" with no conversions to "spend" it on.
I didn't run hard numbers, but it seems like a plausible scenario.