"After importing CAD files to Revit, tens if not hundreds of unwanted Line Patterns (Line Types as per AutoCAD terminology) are created. So every time users wish to select a pattern, the list containing unwanted Line Patterns are displayed. The default purge command doesn't assume that you don't need them, and so, unfortunately users have to delete each of the Line Patterns one by one..
"Many Revit users have started to notice this issue since the new CAD exporting dialog has been provided, with a dedicated table to map Revit Line Patterns to CAD Line types. This tool helps purge all such imported line patterns with a single click."