Why does Ripple use Blockchain technology in the first place? This is what I understand Ripple's use-case with banks.

You and I announce to a trusted third party that $10 million of my money should now be considered yours. This third party isn't a middleman who holds debt,  - it simply keeps track of IOUs and regularly announces to all parties who owes who how much. The ACH system in the US (a.k.a. "Direct Deposit") works this way.

So, for the most part, bank's don't actually send money to each other at all - they just keep track of who owes who how much. Actually settling those debts, if it happens at all, has to happen outside of the systems above.

Ripple, as I understand it, basically a distributed public ledger of IOUs. (And those IOU's can themselves be traded as currency, when they're written by a trusted party.)

But why ripple uses Blockchain technology. Here doesn't Ripple acts as a middle-man? Why not to use central DB's? What is the difference of Ripple from a company that creates network of banks but rely it's technology on Oracle DB and related API's for example? Also who are the miners and nodes of the Ripple's use case with banks?

Thanks.
Firat
This is a very good question. The simple answer is marketing. Technically, a standard database would have been much more efficient in achieving the same. However, there would have been even less reason to invent a token (XRP), you couldn't have found them on coinmarketcap, and they couldn't call it blockchain. So no billions for the founders.