I don't understand these types of thing. I can't imagine it will sell more hardware, but seems like it's something that helps retain who they have in their walled garden.
To understand the need for AppleTV+, you just have to understand how Apple has always functioned, controlling the entire experience: "We make the whole widget" said Steve Jobs famously. Back then, Apple made the hardware, the operating system and they made reference apps while allowing third parties to follow behind and offer their own apps on the platform.
Today, the time spent on Apple's devices, that used to be on apps, has been superseded by a different kind of content: video. And it's not even close. Video content is now by far the number one use for these devices. For Apple to continue to "make the whole widget", they have to move beyond just apps and also produce their own video content on par with the content giants. It doesn't mean restricting others, just offering that reference model that enables others to compete against.
With people spending more and more time inside of streaming services' own apps and user experiences, that UI begins to become what Apple users are exposed to the most. Had Apple not created AppleTV+, those giants would begin to increasingly influence how Apple devices are used and you'd have your Apple experience determined by outside third parties in the same way that Facebook had sought to create an internet within the internet, apps within their own apps, taking over the default experience. For an example of how that was already happening, look no further than the way Netflix had started pushing Apple around and dictating their own terms, preventing Apple from building the TV app the way they had liked to.
With AppleTV+, that upper hand has been levelled. Apple is now an emerging threat to Netflix' model, just enough that they'll have to play Apple's game if they want to continue to compete. I think that looking ahead, we'll have healthy competition with a set of services all offering content in Channels in the AppleTV app, AppleTV+ being just one of them.
