The sheer number is irrelevant, though, if it is difficult to find what you are looking for and hard to browse all that content. And the ability to download what you've bought so it can be watched direct from anywhere means there's no need to be in Wi-Fi range. There's a serious selection of media available in 4K for those who use Apple TV 4K.
ITunes provides access to 50 million tunes and more than 100,000 movies and TV shows. All of these are available through iTunes, and once you have them, you can stream them across your full range of devices - laptops, phones and tablets - regardless of whether they run iOS or Android. It's also about media you don't already have - music, podcasts, films, TV, even audiobooks. And iTunes it isn't just about providing access to the media you happen to already have - CDs you have bought in physical form and ripped, for example. ITunes isn't the only way to manage audio media on your various Apple devices, but it is the official Apple software.
