A guest list belongs to a single event. That suits most cases, but some guests should be on the list at every event. With passes, you create a list of guests once and link it to as many events as you like, and it stays in sync across all of them.
Passes are ideal for recurring guests like members, staff, or season ticket holders. Once you create a pass type and add passes to it, those pass holders automatically appear in every event where you've linked that pass type. Any changes you make to the pass type—adding, editing, or deleting passes—instantly sync across all connected events. Only account administrators can create pass types, but you can grant other users permission to manage them once created.