Server cannot rely on client for storing its own domain's state - while client can store user-related state for improving performance, usability.

For example, twitter cannot rely on a client to store the tweets while the client could cache the recent tweets for improving user experience.

Again, for an app playing music, keeping bookmark of the last song played can be fully stored on the client. Yet if it is needed to sync this bookmark across multiple devices (similar to kindle) it becomes a server concern.