Note the use of the word "traditionally". For the sake of backward compatibility - and the fact that at some point you WILL run out of physical RAM due to the way Windows "manages" it - this is still pretty much the case!
For example, if you had a brand new PC with 256MB RAM, and ran Windows 2000, you would see around 132Mb taken by the Operating system before you started any actual work/game playing.
If you opened Internet Explorer, Word and a couple of other packages, these all create a shed load of temporary stuff that's saved in various caches all over the disk. Much is sent automagically to the VMM, which stashes it in Virtual Memory, since it knows it might not need it for a while.
This is another strand of Virtual Memory - stuff is put there for later use. The same applies to most caches. The Least-Recently Used algorithm looks around to see what hasn't been used recently, and sticks it somewhere out of the way. Recently can be around 50 m/s, depending on the cache in question.
This is so that you still have a large contiguous chunk of real memory to load that 40Mb Wav file - or 100Mb Video file into.
You can never have "Enough" physical RAM.
