You go to a site and register for an event. So these sites want to make sure your customers don’t miss it. So the create links to "Save the Date" or "Create Calendar entry"Event Invitation emails are available to invite your guests.
It’s a good idea to have an “Add to Calendar Link” option for this.. Something like mailto: option on browsers.
Currently most sites has a create calendar invites in the 3 major calendar platforms – Outlook, iCalendar®, and Google® Calendar.