The cache of your iOS device contains temporary app-specific data which is automatically stored by iPhone or iPad apps.
For example, cache data associated with the Facebook app includes the profile pictures of your friends. As a result of the temporary data stored in the iPhone cache, apps tend to be more responsive and to use less network data. However, the cache takes up a considerable amount of space on your iDevice.
With time, cached data accumulates and uses loads of space on the iOS device. Therefore, we recommend emptying the cache from time to time.
What happens when I empty my iPhone cache?
The removal of the cached data has no influence on the functioning of apps. After clearing the cache, it is possible that the loading of pre-cached data takes a bit longer the first time you run apps. In the case of the Facebook app, profile pictures must be reloaded so that they are available on demand. The next time you run the app, however, there is no difference in speed observed.