It’s pretty obvious what the Previous Block Hash does: It merely contains the hash of the previous block.
But notice this: every block does that.
This means that if any block is altered, then the block after will also be altered, and so will the block after that.
Changing any part of this history also changes the entire future after that point.
Here’s an example of what happens when that tampered Merkle Root from before manifests within the blockchain.
It changes the block header, which changes the next block’s Prev Block Hash, which affects the rest of the future block hashes as well.
The rest of the network will reject the history proposed by this blockchain because they will not agree.
This is why the blockchain is considered immutable, because all tampering is easy to see.