This button deletes the cache. Command-clicking will delete the MagicCookie along with the cache.
Click-holding provides the following options:
The button has a safety feature which looks for the presence of the 'CCache log' file before deleting the cache. This makes sure you've set the correct cache folder path with the Prefs button. If the 'CCache log' file is somehow deleted, the Delete Cache button won't work until a new file is generated.