More and more, smart TVs include one of the above-mentioned systems as the "smarts" of the smart TV. You can find Roku TV devices. Those are TVs that include the Roku software. You can also find TVs with Fire TV or Android/Google TV built in. You turn then on, and there is the Fire TV or Android/Google TV menu.
If you have one of those TV -- Roku TV, built in Fire TV, or built in Android/Google TV -- then you really don't need a streaming device attached. Unless of course you want to. More about that in a minute.
If your smart TV doesn't use one of those platforms, then I'm going to suggest that you look seriously into adding one of those platforms to the TV. That doesn't mean you should use, say, a Roku device on your TV. If the proprietary smart TV interface included with your TV does everything you want it to do, then no, you don't need to get a Roku or Fire TV to attach.
On the other hand, I don't know the last time I bought a TV that wasn't a smart TV. 20 years ago? Longer? Something like that. On the last non-smart TV I had, I put a Roku and Apple TV device. That was in 2010. The last few TVs I bought all had built-in smarts. I tried them, but didn't really like them.
You see, I had used Roku and Apple TV, so I knew just how good a streaming device could be. I had experience with a faster, more robust system, and the built-in system didn't come close. If I had not had Roku or Apple TV experience, I may have been satisfied, but since I was familiar, I wasn't going to put up with it, so I kept using Roku, Apple TV, etc., bypassing the build-in systems.
Depending on personal preference, and the abilities of the family members, you may want to consider buying a device even with one of the top tiers platforms on your TV. If your family members prefer, or if you prefer, something like Amazon Fire TV above all else, and if your TV has Google TV built in, then sure, go ahead and add a Fire TV device to it. Or if you really like Apple TV, put that on your TV, even if it's a good quality Roku TV.
Essentially, do whatever makes your Streaming Life run better.