How Does Proof-of-Work Mining Contribute to the Security of a Blockchain?

Proof-of-Work (PoW) mining secures the blockchain by making it computationally expensive and resource-intensive for a malicious actor to alter the historical record. To change a past block, an attacker would need to recalculate the PoW for that block and all subsequent blocks faster than the rest of the network combined.

The massive energy expenditure required to achieve this makes attacks economically prohibitive, ensuring the chain's immutability.