For those with kids, it's a no brainer. Millions of subscriptions right off the bat.
Then imagine if Disney puts Star Wars exclusively on their service, same day or even before theaters. Millions more subscriptions. They could do this with all things Pixar, Marvel and other big Disney IP...
Now add on the merchandise. They'll have better data on exactly what people are watching, how many times they're watching it and can create offers based on that. The merchandising side is a huge advantage.