Probably because the paging file's existence allows the modified page writer to work instead of just piling up memory pages on the modified writer list. Also, it allows more memory commit to be used (commit != actual RAM usage), makes sure you have additional "memory" in a pinch, and (by default) is used if you crash and need a memory dump. If you choose to run without one, that's your choice, but be aware it's not a "free" choice by any means. You ARE messing with how the memory manager works, at the very least.