You do not need to buy a Roku Streaming device (like the Streaming Stick 4k/4k+) if you already have a RokuTV (Roku operating system already built in).
The Roku Streaming devices are used for giving non-smart TVs the Roku OS (thereby making them smart), or other smartTVs that come bundled with another OS (FireTV, Android, etc) the ability to use the Roku OS.
You can, however, use a Roku Streaming device on older RokuTVs to help give them a performance boost or when the RokuTV has an issue and the Roku OS starts to have issues.. Just serves as an extra method to help upgrade older RokuTVs, and give them more life.
Personally, I find my Roku devices to be much faster and responsive than my RokuTV, but others with RokuTV prefer them over the Roku devices and find them just as fast and responsive. If your RokuTV is functioning fine, then I would say there is no need to add a Roku streaming device to it.
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
Insignia RokuTV, Ultra 4660x, Premiere+ 3921, Express 4k+ 3941