It can prevent people from leaving fields blank, from entering too little or too much or from using invalid characters.
But if the user unchecks the box, the user DOESNT have to enter info in textarea.
All you need to do is extend the command, in which case the form will be submitted.
You'll see that the all validation scripts presented on this and subsequent pages adhere to the same basic format.
The purpose of a form validation script is to return a boolean value ( to reference form fields, but that can lead to namespace conflicts and why make things more complicated than necessary.
When the form is submitted - either by hitting Enter or clicking on the Submit button - the to abort (cancel) the form submission. In a real-life situation you will most likely have more fields to check, and more complicated conditions, but the principle remains the same.