The Cartel Coin benefits of having and maintaining a subscription are a bit confusing at the moment, but it'll all clear up as the launch day approaches.
Just as all MMORPGs that aren't named 'World of Warcraft' do, 'Old Republic' is becoming a free-to-play game with a subscription option. As you might expect, playing for free means limited access to content, character creation options, travel, auctions and more. It also puts you on the "normal" list when you're logging in, instead of the "priority" list that subscribers get.
If you subscribe to the game you'll also get additional Cartel Coins – a new currency that will be provided to both free and subscription players on a monthly basis. If you're a current subscriber you'll get 150 per paid month prior to now and 200 for each paid month until the free-to-play launch. You'll also get 250 for playing at all in August and 1,000 for owning the Collector's edition. Confusing things just a bit more is that only some of these options are available for new players.
Those coins get you in-game rewards which look to be purely aesthetic at this point. We expect to hear more about those rewards as we edge closer to release
Source: Old Republic