While it’s true that many of today’s smart TVs come pre-installed with a wide variety of entertainment apps, external streaming devices will almost always improve the user interface of a given app.
Amazon’s Fire Stick, for instance, gives users access to more content with more control than the standard Amazon TV app, with other products in the Amazon line even coming equipped with Alexa – giving you the extra option of controlling your TV with your voice.
It’s also rare that a smart TV will come loaded with access to every streaming service out there. Apple TV Plus, for instance, seldom comes as a pre-installed app on standard smart TVs, hence the need for Apple's 4K HDR streaming box, while Panasonic TVs don’t support Disney Plus.