One of the big advantages of Ansible, compared to a bash script, is its idempotency, ensuring that everything is in order. This is a very nice feature that not only assures you that nothing has changed the configurations on your server, but also that new configurations will be applied in a short time.
Due to these reasons, many people run their master.yaml file once a day. When you do this (and probably you should!), you want some kind of feedback sent to you by Ansible itself. There are also many other cases where you may want Ansible to send messages to you or your team. For instance, if you use Ansible to deploy your application, you may want to send an IRC message (or other kinds of group chat messages) to your development team channel, so that they are all informed of the status of your system.
Other times, you want Ansible to notify Nagios...