Database Caching Class

[eluser]al pacino[/eluser]
If the database itself is handling caching faster than a file based system could,then what is purpose of using "Database Caching Class"?
"When caching is enabled, the first time a web page is loaded, the query result object will be serialized and stored in a text file on your server. The next time the page is loaded the cache file will be used instead of accessing your database. Your database usage can effectively be reduced to zero for any pages that have been cached."
It is given in codeigniter user guide.
When I opened these cache files,i can see result object stored in it.So,this is working fine.
But,next time when page loads i don't think it is using cache files.Because,changes made to the database are reflecting on that web page,which should show me the old data stored in cache files.
Thanks for the reply...