If you live here, bank accounts can be very useful. Even if you take money out of your ATM and deposit it in your local account every now and then (like I do).
For example, if you go to WalMart and use an international debit card you will have problems. Your card cannot be used by the normal register credit card systems. If you don't tell the cashier that your card is international, they will just tell you it has been rejected. If you tell them, then they have to go to a special machine on one of the higher-numbered registers to process the card and you will wait. Not a big deal, but nice to not have to deal with that.
DirectTV, who used to allow foreign cards to be used (I don't know if it's a problem with credit cards versus debit cards) for things such as the original deposit to get the installation and boxes dealt with no longer does. You have to go to Santander Rio, make a deposit in the machine to their account, take the receipt and fax it to them with your account number they gave you to get it started. As I found out, that process can have some drawbacks.
Other services will allow local bank accounts to be used but not international cards to pay the monthly balance.
Most (all?) local banks have deals with local providers on certain days to provide good-sized discounts if you use your card on those days at those stores (not to confuse with Disco and Coto, who also have a certain day where just using a card gives a discount, and I think international cards work with that).
I have a local bank account and really enjoy its use. I don't have a DNI yet, but my lawyer has a contact at Banco Galicia who allows an account to be opened for those going through the residency process. I've heard that you don't need a lawyer necessarily to get a bank account for the residency process, but I have to think it's probably easier, having tried unsuccessfully to open a bank account before my lawyer did it.