Use Facetime as an example. If it's also available to Android and Windows, I will be able to use that with more friends. I will be more satisfied as an existing customer (hence more loyal), and my friends on Android / Windows will be able to see the superb video quality and it may become a factor of conversion eventually.
Also, if Page and Keynote are available on Windows, since they are significantly cheaper and probably have all the functionality for majority of the users, it could take bite of the Office market share.
If FaceTime is available on Android, why does your next tablet need to be an iPad? You know you'll still get the same video quality using a cheaper Android tablet, after all.
Also, if Page and Keynote are available on Windows, since they are significantly cheaper and probably have all the functionality for majority of the users, it could take bite of the Office market share.