The example I gave was contrived. I frequently find myself modeling much of my logic as a series of data transformations. The more I can model logic like this, the easier it is to test.

For what it's worth, your example doesn't map 1-to-1 to the intention of what I wrote. Doing it in a more imperative style, I'd have to write

    emails = list()
    for user in users:
        emails.push(user.email)
    res = sendEmail emails
    if res.IsError:
        Error("Error sending emails")
    else:
        res

(excuse my pseudo Python)