When you have multiple accounts, you have access to multiple streams of money. If the US government infiltrates one and steals your stuff, chances are that they won’t get everyplace at the same time. There will be some warning. So if something of yours gets seized, you’ll be able to live off of one of your other accounts while finding a lawyer (or, if we continue going the pre-WWII Germany route, getting your ducks in order to flee the country). If, say, Bank of America is your only account, then you’re going to run up a lot of credit card debt (or worse) while waiting for things to be sorted out. If you can get them sorted out.
Now, of course, it doesn’t have to be fascism that causes inaccessibility to one of your accounts. Sometimes your account will get compromised by theft or there will be some kind of clerical error that freezes an account. When DH’s debit card got spoofed and our Wells Fargo account was temporarily frozen, I was grateful to have access to a checkbook from a credit union account with money in it to pay the monthly bills.
How would you access money if your main account got frozen?