Cron automatically sends an Email message containing both stdout and stderr outputs of any executed command. The "Prevent Email" button just helps getting rid of that message by suffixing the command with ">/dev/null 2>&1", which is how you redirect the stdout and stderr to nowhere in a shell.
If you set a cronjob and you don't want to receive any message containing the output of the executed command, click on it. In you want to receive it, don't click on it.