Each time you (save and) publish a workflow you should be able to enter what has changed in a textfield. If multiple people give support for a client and they have to check the workflow designer to see what has changed, it'd be nice to know what happened in each version instead of manually checking the current and previous version.

If the 'shape' of the workflow is the same but the contents of some activities have changed, you have to check all activities of both versions one by one to find the difference.