There are many, but here is a personal anecdote. For background, I am a relatively high income software professional. Many years ago, I moved states, and in the process had to switch banks, so I closed my old account before moving.
Somewhere there was an automatic debit on one my bills for 30-50 US bucks that my bank paid, but since my account was already closed, it basically went into default. When I tried to open a bank account in my new state, I was denied.
It ends up there’s something like the credit reporting agencies, but for banks, and because of my $50 dollar accidental default, I ended up on this system. There are almost no consumer protections on it, so I couldn’t contest it, and I didn’t have any way to pay it or get it removed.
The end result is that I was unable to open a bank account for the next 5 years without a co-signer, even though my credit score was in the 700s and I had a six figure income. I was able to get credit cards, but couldn’t easily pay the bill without an account to write checks from. I had to go to check cashing places to get money orders (most banks won’t give you money orders or cashiers checks without an account), I had to cash my paychecks at the one office my employers bank had in town, and I could only receive checks for banks I could travel to. When I rented my apartment, I paid the security deposit in cash, but then had to drive around town when I got it back after moving out to try and find a branch of my landlords bank while I was on my way out of town. It was a nightmare.
There’s a whole system I fell into that I wasn’t even aware existed until that happened to me, and I consider myself a highly educated consumer compared to your average person. There are plenty of reasons to want cash in our society, even if you are not doing anything illicit or that you want to keep hidden. If, for any reason, banks are not available to you, life gets very difficult very quickly.