One of the selling points of the Apple Watch is being able to use it (though not own it since it needs an iPhone at some point) in isolation. Having to stream content isn't always an option or isn't the best option. I use mine on my runs, but it has no cellular data of its own so I downloaded music and audiobooks to it. Even if it did have cellular data, streaming the music would drain the battery even faster since I'd be using every capability: health stat monitoring, bluetooth, GPS, and cellular data.