It was one of the tools that allowed me to maintain healthy credit card usage habits. In my idle time when messing around on my phone I’d gotten into the habit of just transferring money from my checking account to my credit card, often in excess of my actual balance, so as to cover “pending” purchases.
Not only will my bank no longer let me do that, just to twist the knife a bit, they will let me make multiple payments totaling the pending total due so long as no individual payment exceeds my actual balance.
So it’s not that they can no longer handle negative balances on the card, it’s just that they’ve half-ass made the interface disallow it.
It's a common financial instrument. You can find many providers online.
You get a much better interest rate than a typical savings account (I think current rates are around 4% or so), but you are typically limited in the number of withdrawals you can make per month. Banks offer better interest rates because the cash is less volatile and sits in the account longer. They are good for cash you want to park somewhere for a while but that you still need quick, infrequent access to, like emergency funds. Credit card payments are also a fine use for it, though I'd say the benefit is pretty minimal unless you consistently maintain a balance greater than the credit card payment.