It could be useful to stop the Submitting process, in case you find some problem with data (e.g. on Quote inputs, etc)
When you hit the Submit button, it executes the Workflow logic. But if you raise exception in the WKF logic, it will stop the submitting, and go back to Draft.
api.throwException("There is some error in Quote, please fix it before submit")
If this is in Workflow logic for Quote, then the message will appear in the Message portlet window of the Quote