HTTPS allows the visitor to verify the identity of the website he is accessing, thanks to an authentication certificate issued by a third party authority, reputed to be reliable (and generally part of the white list of Internet browsers). 

It theoretically guarantees the confidentiality and integrity of the data sent by the user (especially information entered in forms) and received from the server. 

It can be used to validate the identity of the visitor, if the latter also uses a client authentication certificate.