It’s like this.
If you want a variable to survive between games, it has to go in the save game.
In can just live in the save game. If you’re only accessing it every now and then, why not just leave it there, and just update it in the save game. Because you know it’s always saved, and you get at it from any blueprint.
But, if you’re changing the variable a lot ( like on tick, or as part of a calculation ), then you need to get it out of the save game, and only put a copy back when you need to. Otherwise you’ll be hammering access to the save game.