Strongly authenticating users is critical in securing any infrastructure. The potential for mischief increases substantially if you cannot be sure with whom you are dealing. Ensuring the integrity of data transmitted from user to user helps prevent the data from being changed. Technical non-repudiation binds a user to a transaction. This will provide important forensic evidence in the event of a later problem.
Encryption protects private information from being divulged even over open networks.
In a typical PKI, two key-pairs are generated by or for each user. Each key-pair comprises two keys. In each key-pair, one key is kept private, and the other made public. A trusted party cryptographically binds the public key to the person's identity by digitally signing the certificate. The digital signature on the certificate ensures that any unauthorized alteration of either the identity or the public key will be detected.