I know this is an older post, but it still comes up in searches.
The SaveGame is used by FArchive::Serialize() internally when ArIsSaveGame is set. Any variables on the blueprint that have SaveGame checked will be serialized. I’ve done all this in C++ where I call Serialize() in a C++ class that blueprints inherit from my C++ class and then the archiver will check for the SaveGame being checked on properties in the blueprints and will also serialize those out to disk as well.
I’m not sure if you can directly use SaveGame in a blueprint only save scheme.
See this documentation about SaveGame property:
https://docs.unrealengine.com/latest/INT/Programming/UnrealArchitecture/Reference/Properties/Specifiers/SaveGame/index.html
See this documentation for saving games in blueprint or C++:
https://docs.unrealengine.com/latest/INT/Gameplay/SaveGame/index.html
For tips on using the archiver in C++ definitely see this post and read the Fortnite portion: