> Why are these files retained?
So that you do not need to download them again next time you visit the page, but if you visit pages like the expatboard that is not so smart, so depending on how you have setup your internet options the files will be used in different ways.
Look in control panel internet options/ general/temporary internet files/
(this is for win2000 not sure if it is the same on win95/win98/winMe/Nt but it should be simmilar). There you should have a delete files button that will remove all your saved files.
On settings you should be abel to set how much disk space to allocate for saved files and set a option specifying if a page should be downloaded or taken from the saved files on your disk. It is called "Check for newer version of stored pages", depending on how you surf and what kind of internet connection you have choose a appropriate setting.
Hope it helps!