One example use case I saw recently in my circle of friends: person A tweets "wow, there's a great special on sashimi bowl at the place near the university", person B tweets back "@a hey, I'll be in the area Friday, how about we go for lunch? anyone else interested?", person C (common friend) sees the conversation and tweets "@a @b hey, I'll go too". Finally a group outing is quickly organized.

Mostly I see it used for quick conversations, funny remarks, organizing small events, saying when you're going for a trip...