Broadcast emails are something you'll never use, but it's a way to send a broadcast to your entire database via email. If ever you're in a bind and need to get a message out quick, that's what it's for. You can launch them the same way you launch an SMS broadcast, but make sure to select "Email" in Step 3 when creating or editing a broadcast.
You can also select "Both" if you want to send both SMS and Email broadcasts at the same time.