I could easily imagine someone unaware of this hook running a test to create a bunch of user entries, sending emails all over the place without even realizing.
It's like someone read http://en.wikipedia.org/wiki/COMEFROM and took it seriously.
You could imagine callbacks or observers that do all sorts of things:
ping an external service
touch another model or models
send an internal email
or does a combination of a bunch of stuff.
We could just as well be talking about S3 data, facebook posts, tweets, anything with (side) effects beyond the active database record in question.