We should generally see hashes as a commitment to some data. Imagine we have a huge table of data. If we hash all of this data together (in any format), this hash is essentially a fingerprint for the entire data.
Now this commitment can be used to be shared across a network. Anyone who knows and trusts that the commitment hash is correct, and quickly verify if their copy of the data (possibly coming from an unknown source) is valid or not.
Block Header contains this very same system, except it uses a