You do not need to activate cross save if you do not have the other platforms. It is only intended for use if you plan on playing with your account on another platform. If you do decide to play on another platform, you have to create an account (such as stadia for example), and link it to your profile. After you have your accounts linked you can then activate cross save. You can find more information here. https://www.bungie.net/7/en/CrossSave