Hey there. Cross-save lets you share characters and progression from one platform to others. If you only play on one platform, then cross-save is completely unneccessary, the whole point of it is for people who game on multiple platforms. So it wouldn't make sense to set it up if you only play on Playstation. Why do you want to set it up, exactly? Is it to do with the Stadia shutdown? That affects people whose characters originated on Stadia. You don't need to do anything if you don't use Stadia.