Zcash privacy: Linking a transaction to a note

From the zcash protocol document: “The basis of the privacy properties of Zcash is that when a note is spent, the spender only proves that some commitment for it had been revealed, without revealing which one. This implies that a spent note cannot be linked to the transaction in which it was created.”

I fail to understand why the above is important. If an adversary can link the transaction to the commitment on the Merkle tree how does that help him in any way since the sending and receiving address are shielded?