Callback plug-ins mostly control the output we see while running CMD (Command Prompt) programs. Apart from this, it can also be used for adding additional or, even, multiple outputs. For example, the log_plays callback is used to record playbook events into a log file and the mail callback is used to send an email on playbook failures.
You can also add custom callback plug-ins by dropping them into a callback_plugins directory adjacent to play, inside a role, or by putting them in one of the callback directory sources configured in ansible.cfg.