Magento does not clear old cache entries by itself, therefore the cache will grow and slow down the whole application.
The Aoe_CacheCleaner will clean up old cache-entries by firing Mage::app()->getCache()->clean(Zend_Cache::CLEANING_MODE_OLD); in an user-defined interval.
Additionally you're able to clean up other caches as well, such as the Catalog Image Cache and the JavaScript/CSS Cache.
http://fbrnc.net/blog/2011/02/magento-automatic-cache-cleaner