Most people have a line of credit far larger than their actual use. With a debit card that's coming right out of your account, which means your mortgage, if it comes in before you notice the issue, is going to then fail.

This is also partially that people don't put critical bills on their credit card typically. And even if your credit card does get maxed out, you typically would have a second credit card handy. But those credit card payments have to come out of your bank account and so you're risking that you intend to pay your credit card you said Set whatever it is to send the money in but there's no money in your account And so it doesn't get paid and now you have late fees on other accounts

If your debit card is going to a different bank than what you normally pay all your bills out of, this is not a worry. That is not how most people I know handle their banking though, which is why it is a real problem to worry about.