There are two main reasons for ensuring integrity. The first would be to hash data stored on a file server so that it can be proved whether or not that the data has been tampered with. This could also be the case for a forensic examination of a laptop seized by the police—the forensic scientist could hash the data before the examination and then re-hash it at the end to prove that he had not tampered with the data. The hash values should match.

Another method of proving integrity would be to digitally sign an email with your private key to prove to the recipient that it has not been tampered with in transit. Prior to this, you would have to send them your Public key to validate the email. This proves that the email has maintained its integrity and has not been tampered with in transit.