Sandboxed applications have certain document folders where they can keep data such as plists and sqlite files. Those folders are resistant to upgrade, meaning — when you upgrade the app from the app store, the data will not get reset.
However, when you remove the app, all local data is removed? isn’t it? is there a way to persist it beyond application deletion? so that if you re-install, you just get your old data back?
You can store data externally in iCloud, Dropbox, or another online service.
The nice thing about this is that not only will the data survive deletion-and-reinstall, but it will also survive the user replacing the device, upgrading to a new device, sharing data between devices, etc.