What's the difference between 'commerce-line-item-unchanged' and 'commerce-line-item'
Commerce line items are Drupal Entities (enhanced a bit by the Entity module) and, like other entities, the "entityname-unchanged" is present in some rules just so you can see what the entity was like before it got changed.

There isn't a specific use case intended for it but there are cases where being able to see the original state of an entity would be good.  Specific to a commerce line item, maybe you'd want to reset the line item's quantity to its old value if they chose some crazy number like 10000.