There are several triggers you can use for sending timeline updates:
- Location updates
- User actions (share/custom/reply)
- Some "external" trigger coming from your service
But there is one use-case (that I think is rather useful) that isn't covered by those triggers: User triggers an action and you want to send them a series of timeline cards/updates on a certain schedule.
Anything timer related falls into this category ("This card will self-destroy in 10 seconds") and I can think of several examples where this can be useful.