What is the purpose of the hook function?

A hash function is used to transform an arbitrary set of data into a fixed-length string (hash or digest). Main purposes:

Data integrity verification: By comparing hashes before and after transmission/storage, data changes can be detected.
Secure password storage: Instead of storing passwords themselves, their hashes are stored. During authentication, the entered password is hashed and compared with the stored hash.
Digital signature: The hash of a document is signed with the sender's private key. The recipient verifies the signature with the public key and compares the received hash with the document's hash.
Data structures (hash tables): The hash of a key is used to determine the index or address where the corresponding value is stored, enabling quick search and access.
Cryptocurrencies and blockchain: Block hashes are used to link them in a chain and ensure data immutability.

Key properties of a "good" cryptographic hash function:

Deterministic: The same input always produces the same hash.
Efficient computation: The hash is easy and quick to compute.
Collision resistance: It is very difficult to find two different inputs that produce the same hash.
Preimage resistance: It is difficult to find the original data given its hash.
Second preimage resistance: It is difficult to find another input that produces the same hash as a given input.