When reserving addresses in a process, no pages of physical memory are committed, and perhaps more importantly, no space is reserved in the pagefile for backing the memory. Also, reserving a range of addresses is no guarantee that at a later time there will be physical memory available to commit to those addresses. Rather, it is simply saving a specific free address range until needed, protecting the addresses from other allocation requests. Without this type of protection, routine operations such as loading a DLL or resource could occupy specific addresses and jeopardize their availability for later use.
