It seems like an odd omission but Google Play Movies and TV on iOS used to not let you watch shows or movies offline. At least Google finally address this issue with a new update. You can indeed save the shows and films you’ve purchased on Google Play Store on your iOS device.
The app is still pretty limited though because you can’t buy or rent stuff on iOS. You can only watch those you’ve bought on an Android device or through Google Play Store. This just might come in handy for those who switch back and forth from their Android and iOS devices.
Source: The Verge | Download: Apple App Store (Free)