For example, perhaps you have received an email with the hundreds of email addresses that were cc-ed on the original email, and you have to scroll through them to get to the message. It's annoying. If it's not necessary for the recipients to know who received the email—maybe it's just a reminder that the office is closing early today--and you want to avoid the scroll through the cc's, then you can bc all the recipients. That way, there's no cc scroll, and when someone replies, there's no chance that he or she will accidentally hit "reply all."