It's neat. tried it once when I read about it yesterday.
Seems like it'd be handy for when I'm trying to intercom the wife in the living room and it turns out she's somewhere else.
Then I could say "hey, call me in the office when you get a chance" and she'd probably hear it unless she's in the one location I can't think of that doesn't have an Echo.
For a big house/family, it's most obvious use is "hey, it's dinner time!"