I don't understand the hole of oe_form_dirty use in web_form.js

Any body explain what is purpose when use this option?

In view_form.js

// If the form has not been modified, the view can only be valid
        // NB: is_dirty will also be set on defaults/onchanges/whatever?
        // oe_form_dirty seems to only be set on actual user actions
        if (!form.$el.is('.oe_form_dirty')) {
            return true;
        }