Windows 7 - Thumbnail Cache

Ive got some questions about the Thumbnail Cache, excuse me if I'm in the wrong topic I wasn't sure were to put this.
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Windows Disk clean up and Ccleaner both clear the Thumbnail Cache, why is this? Why would it be good to clear it out? Also what exactly is the Thumbnail Cache?

Thanks!
Zenix Net

Two tutorials by Brink
Thumbnail Cache - Clear and Reset
Thumbnail Cache - Enable or Disable

thumbnail cache loads a frame of video, or a preview of whatever filetype is set to use thumbnails and stores it somewhere (caches it) so that it doesn't have to be loaded every time a folder is viewed so the file doesn't have to be accessed every time and resized to a thumbnail (reading a file and resizing the snapshot of it can take up lots of ram and cpu threads if you have a folder full of media with thumbnails). if your editing these files alot, its good to refresh the thumbnail when you need to or clear the cache. i personally prefer detailed view rather than thumbnails.