Single payer doesn't mean everything is covered, and the Sanders plan does have premiums of 8.4% of your income, in addition to much higher income taxes and taxes on interest and dividends. HSA's could be used to pay those premiums or for optional healthcare services. Maybe you want a dental cleaning three times a year instead of twice, or there might be fees for non-emergent use of the ER, or convenience fees for urgent care... lots of possibilities.
Source:
https://berniesanders.com/issues/medicare-for-all/A 6.2 percent income-based health care premium paid by employers.
A 2.2 percent income-based premium paid by households.
Taxing capital gains and dividends the same as income from work.
37 percent on income between $250,000 and $500,000.
43 percent on income between $500,000 and $2 million.
48 percent on income between $2 million and $10 million.
52 percent on income above $10 million.