A consensus algorithm is a process implemented in distributed processes or systems to achieve agreement on some particular data. The blockchain consensus algorithm keeps the ledger transactions synchronized across the network to ensure that ledgers are only updated when the appropriate participating board approves transactions and, when ledgers are updated, they are updated with the same transactions in the same order. This process is called a consensus, and the protocol program that keeps this running is the consensus algorithm. That is why a consensus is considered to be the running engine or heart of blockchain. 

So, consensus primarily establishes a strong technology infrastructure layer for the blockchain, which makes it the most critical part. It ensures that every next block that gets added to the blockchain is the one and only true version and that there are no other blocks that represent another version. It also safeguards the entire blockchain against powerful adversaries that may potentially derail it and cause it to fail to maintain its integrity. In short, for a blockchain network, achieving consensus ensures that all nodes in the network agree upon the consistent global state of the blockchain.