When you've subscribed before you get bonus cartel points for that when you resub or maintained your sub. Even though you don't need any of the account upgrades and such you can still spend cartel points on armour suits or cartel packs which can give some nice items. The packs give random items but some of them are quite cool or useful and can be traded in game after a cooldown. Armour sets are adaptive gear for example.
You can also choose to buy legacy upgrades or extra storage panes with cartel coins instead of using credits. I unlocked the rocket boots for example with cartel coins.
With your sub you also get 500 cartel coins every month from now on...so hey. I'd use em for legacy upgrades and cartel packs myself.