If something is ‘notarised’ on a blockchain, we can guarantee that the data previously entered is still intact and has not been altered. All you need is a copy of the original data (a file, a log file or a record) to be able to recalculate its hash values at any time and check that it matches the content saved on the blockchain.
Plus, not only can you prove that the data has not been altered, but that it already existed when it was ‘notarised’.