Save game serialization is different and is handled manually, UE4 doesn’t provide a way to serialize runtime state of whole actors out of the box.
Transient disables serialization of a property when the containing object (in this example a character) is serialized to/from disk, which generally means when saving/loading the character blueprint in the editor, or loading it at game startup. A CurrentHealth property has no meaning in those contexts, which is why it would be made transient.