It's obvious why this is useful: if you need to fill the same form every year (say, because it is a tax form) you don't want to retype the same information again. Importing data from a previous submission will save you time.
This feature works out of the box for forms embedded in client portals as long as the form has the "multiple instances" flag turned on. To enable it, go to Portal Group, select Form templates, and enable the option for an appropriate form:
Then, make sure that the workflow template which includes the selected form allows multiple instances as well:
When a user creates a new instance of a workflow which he or she completed previously a 'copy from record' option will become automatically available.
