Addins add a GUI element to ArcGIS and increase usability.
- More sophisticated GUI elements, such as drop-down comboboxes.
- The ability to communicate between addins, linking several GUI elements together.
- The ability to respond to Arc events beyond the GUI with application extensions.
- Access to the elements of the pythonaddins module (as with other script tools).
In total, these make addins significantly more useful than standard scripts.