Many "cable channels" do have direct apps on Roku. And sometimes those apps allow you to pay them directly for an individual subscription. However, some "cable channels" like CNN prefer their current cable model - you can tell when you see "CABLE OR SATELLITE SUBSCRIPTION REQUIRED" in their channel store description page - like you do with CNN. In this case, the CNN app on Roku expects you to tie it to an existing subscription, such as cable, satellite, OR a streaming TV package that acts as a substitute for cable such as YouTube TV, Sling, Hulu Live TV etc.
Since there is nothing mutually exclusive about Roku and cable, I would suggest you just get one and start using it. Once you've actually used it for awhile, questions like these will become easier. You may decide to use Roku to add to cable or replace it.
Banned but back. Because why not?
