It's rationalized by the vendors as a service to the customer. The mobile app needs to be able to configure the device via the cloud, so increasing the ability for said device to reach cloud by whatever means necessary is a customer benefit.
And of course: If it exists, it can be used.
That said, I haven't seen any evidence that suggests that televisions and streaming boxes are using it.
But remember, too: Whispernet.
Available as a one-time extra-cost feature on the first Kindle back in '07, Whispernet provided a bit of slow Internet access over cellular networks -- without additional payments or contracts or computers.
And really, Whispernet was great in that role.
But the world of data is shaped a lot differently these days. Data is a lot more-available and much less-expensive than it was back then, ~18 years ago -- and codecs have improved by leaps-and-bounds in terms of data efficiency.
Radios are also less expensive and more-capable compared to what they were in '07.
This will be sold as a feature: "Now with Amazon Whispernet, your new Amazon Fire TV will let you stream as much ad-supported TV as you want! For free! No home Internet connection or bulky antenna required! Say no to monthly bills and wanky-janky setups, and say yes to Amazon Fire TV!"
The future will be advertising. (Always has been, but always will be, too.)