I've used Plasma Bigscreen a bit, so I have some thoughts on this.

First, if you don't care about 4K streaming, you can have a perfectly good experience using commercial streaming services through a browser (on x64, at least). I set up Firefox to open fullscreen by default, then created a bunch of .desktop files as shortcuts to each streaming service I used, all opening in their own separate tab containers. That allowed my non-technical partner to open (for example) the Netflix "app" by clicking the desktop icon, which opens to the Netflix homepage full-screen, logged in and ready to go. The few times I've had to use Netflix via a smart TV or Android native app, it has been a worse experience than my in-browser bodge job - same has gone for Disney+, Amazon Prime Video, and even Spotify. What Plasma Bigscreen brings to the table is laying out those desktop shortcuts in a way more familiar to smart TV users - you can create categories so all the streaming shortcuts show up in the same row together, and customise their icons & backgrounds. I don't really need it for myself, but when anyone else tries to use my TV, the familiar structure of the interface makes it easier. It's a small thing, but it's nice.

Second, you can do a lot more things than just stream video when you have a computer hooked up to a TV. Games are an obvious choice since you can use a controller to navigate Bigscreen, then jump straight into a game. Steam (especially in big picture mode) is the obvious one, but I have also found that Retroarch is seamless under Plasma Bigscreen, and with a knockoff air mouse remote you can have a good time even with games found in distro repos like Simon Tatham's Portable Puzzle Collection, or the Space Cadet Pinball demo. Browser games are similary straightforward, and you can have direct shortcuts to them using the same trick I described for the streaming services.

The last thing I want to point out is that Bigscreen is just another desktop environment - you can install it alongside regular old Plasma (or GNOME, or whatever) and switch between them by logging out and selecting the one you want. Having a computer that's hooked up to a large display and can give you a choice of interfaces suited to both TV and desktop use is more powerful than either option alone.