Firestick, Roku, etc add the smart to an otherwise dumb tv. It can't supply any more resolution than the tv has natively. Nor any better contrast or brightness of anything else. It does, however, substitute for the tv's built in tuner and may render a stream faster.
As an extreme example, a 720 dpi tv has a max resolution of 1280x720. A 4k capable Roku or Firestick can deliver at 3840x2160, but the tv can't display the extra pixels, so it's just wasted bit and processing.
The stick device may have some built-in streaming apps or even games that some smart tvs may lack, e.g. Peacock, Paramount, Netflix, Hulu, etc. But these days most smart tvs have most every popular app or can readily download them..
The net is there is little point in adding a stick device if you buy an already smart tv. Only pay for what you need. On the other hand, you could buy a really dumb display (aka monitor) and a Roku or Firestick and probably have everything you want except OTA (and you can buy a tuner for that too).