A blockchain is nothing more than a data structure. It’s essentially a linked list using the hash of the previous block. Distributed consensus is something blockchains are useful for, but it doesn’t define it

Because it’s an immutable ledger, not just a database. It maintains a history of every previous transaction/entry. Blockchains are used by banks and in the supply chain because it makes backtracing and identifying discrepancies trivial. For things like cryptocurrency, blockchains allow “don’t trust, verify” but for something where you already have trust, they allow “trust but verify”