Considering just how little most people have in reserves (in my country the average is something like €500, which is fking ridicukous IMHO), all it might take is, say, your electricity company to make a mistake on a direct debit and all of a sudden there might be hundreds of euros there less than what you expected.

(Sure, they'll refund you the excess money they took ... eventually ... but they won't refund you for any problems resulting from their "mistake", so you're SOLO on that).

Worse, in my own experience in the only time I had one of those (I had moved to another country, was verbally confirmed by the bank when I openned my account that such payments would bounce and yet one didn't bounce and I got charged "unarranged overdraft" fees), even if you have another account with the banks (such as a savings account) with about 1000x what was needed to cover that amount, they won't use any of that money and instead treat it as an overdraft and charge you for it a lot if unarranged.

PS: I ditched that bank within a week, so the £20 or whatever they made from that little stunt was a lot less than they lost, because a lot of money went through my bank accounts (some staying there for long periods until invested) in the years after that.