Not only are they stored but they duplicate - NO WAIT - at least triplicate themselves into other areas of your system! This all adds up to system space usage - anything stored on file (even duplicates and triplicates) take up space (megabytes) on your system.
If you don't practice regular clean-up maintenance on your computer, then information will keep building up and taking up more of your systems megabytes. The effects of neglect are a significant slowdown of your system, with noticeable slowness of internet browsing.