First, know that I am a fan of streaming devices. I think Roku, Apple TV, Amazon Fire TV, and Google/Android TV devices are good options for streaming. But what if you already have a smart TV? Suppose your TV already has apps. Should you get a Roku, Fire TV, or other device? The answer to that question is easy: maybe.
Many 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 TVs -- 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. Why would you want to? Well, there is actually a good reason, which I'll cover shortly.
If your smart TV does not use one of those platforms, then I'm going to suggest that you look seriously into adding one of those platforms to the TV.
Sure, a lot of people like the WebOS (LG TVs) or Tizen OS (Samsung TVs) interface. I don't. If you have one of these, and like it, then you're good. You don't need anything else. Well, probably.
For me, the last time I bought a TV that wasn't a smart TV was around 20 years ago. Maybe longer. On the last non-smart TV I had, I put a Roku and Apple TV device. That was in 2010 (it was already on older TV). 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.
But what if you have smart TV that is one of the major platforms? Should you put a separate streaming device on it? Maybe.
Once, I had use of a Roku TV for about a year. It was an okay experience, but I really didn't care for it. I was used to a better quality Roku device. This was a cheap Roku TV, and it was sluggish compared to a Roku Ultra, or even a Roku Stick. So, I put a Roku Stick on it. I liked the Roku interface, but the device was not performing as I liked. I had a Roku Stick, so I put it on the Roku TV. So I converted my Roku TV to a Roku TV.
In a bedroom, I have a good quality Roku TV. Excellent TV. Great picture. Responsive Roku interface. But as I was testing Google TV, and I put a Chromecast on it. I was using Chromecast on my primary TV, but to fully try out Google TV, I put one on my second TV as well. I wanted all the TVs to use the same interface.
So, 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, or even Roku TV, 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, Android/Google TV, Fire TV, WebOS, Tizen OS, or any other platform.
My Streaming Life is easier when all my TVs use the same interface. You may or may not care. Essentially, do whatever makes your Streaming Life run better.