You must be thinking of Rick's cash tips: https://www.ricksteves.com/travel-tips/money/cash-tips
where Rick recommends:
Bring along some US dollars. While you won’t use it for day-to-day
purchases, American cash in your money belt comes in handy for
emergencies, such as when banks go on strike or your ATM card stops
working.... If local banks don’t have exchange services, you can
always find exchange desks at major train stations or airports.
Rick's intent is converting the USD to local currency as a last resort if the usual means of obtain local currency fails. US dollars are the primary reserve currency, the Euro being the next most popular reserve currency. Last resort in the sense that you will not get as good an effective exchange rate as your ATM cash withdrawals or using your credit card.
$200 or $300 USD would get use enough pocket cash (assuming your credit cards work for big expenses) until you can resolve your ATM card failure. (I carry at least two ATM cards/accounts).
PS Assuming you are flying from/to a US (or other North American) airport, it's useful to have US dollars for small purchases at the US airport. It's hard to spend your Euros in America.