Passive Aligners after Active Aligners during Invisalign ?

Today I came into my orthodontists for a checkup and new aligners. We were going over my clinic heck and I told him I couldn't wait to be done after tray 18, currently on tray 13. He said I had 5 passive trays after tray 18. He said all my active trays would be done at 18 and I would have 5 passive trays then get my retainer. I've been reading online and I don't really get why I need 5 trays of passive trays. It's weird because it lines up exactly one year from when I began.