The event lands in the guest's calendar, and they can subscribe to your whole lineup.
Google Calendar, Apple Calendar and any app that supports iCal. A guest adds a single event with one tap, or subscribes to your whole lineup, and then new events appear in their calendar on their own.
The first is adding a specific event. The guest taps a button in the ticket or on the event page, and it lands in their calendar with the time, place and a link.
The second is more interesting for you: subscribing to your lineup. A person subscribes to your venue or organization calendar once, and every new event appears for them automatically, with no mailing. It is a channel that depends neither on social media reach nor on whether someone opens an email.
A calendar reminds people on its own, at the moment they chose. An email competes for attention with dozens of others and arrives when it suits the sender. For an event someone bought a ticket to a month ahead, that is the difference between showing up and forgetting.
The subscription link is cryptographically signed, so it cannot be guessed by changing a number in the address. That means it cannot be used to read someone else's lineup.