Shoutbox

The short explanation is that programs often request large blocks of memory that they never fully use.  Windows manages memory in 4-kilobyte pages.  For certain types of allocations, the memory manager will allocate all or part of the request in virtual memory.  If these pages are actually ever accessed, they are then swapped in.

This creates a slight overhead for programs that allocate large blocks of memory and use all of it, but that is a very small  hit compared to quickly filling up all of the physical memory and then having to swap pages out to make room for something else.