Can the app be used to clean the disk?

No, because of iOS sandboxing, apps can only access their own data and don't have any access to other files on the disk.

Some apps on the App Store however offer this feature and "clean your disk from junk and temporary files" together with some really fancy animations. What they do is the following - they try to fill the disk completely (usually the iOS message saying "Storage almost full" appears). In this case, iOS tries to free all temporary files and all caches to make more space on the disk. After this, the app deletes all the files it has created and there is more space on the disk thanks to the iOS file deletion. This is a really stupid thing to do:

1. If you run out of space, iOS frees the storage in the very same way automatically so there's no point in doing it manually.
2. Since the caches are deleted, all the cached stuff will have to be reloaded from the network again so e.g. Safari pages will load more slowly after such a "cleaning" and you'll spend more money for your mobile data.