because a computer will never use the extra.
Your computer really is at 90% all the time unless you run Windows XP with 8 GB of RAM. The system uses extra RAM for disk cacheing, unless it is needed for applications. For example, my server computer has 12 GB although only 1.3 GB is truly "free" (not used for anything at all):

You're right; the OS still has 4100 MB available for applications. But 2800 MB is cached. Let's get rid of 4100 MB, and let's assume the OS now has 9100 MB, which is enough to avoid a blue screen. Even though the OS has enough memory to run the existing programs, it will be a little slower since the contents of the 2800 MB piece are now stored on disk instead of RAM. The casual user may not notice the slowdown, but disk caching does help the operating system.
It is, of course, impractical to add 8 GB of RAM to this computer and make it 20 GB. But it's helpful to have a little more than what your programs alone take up.