If WINVER is not defined, it will default to 0x0500, which is Win2k, see windef.h
WINVER is used to enable additional structure members for higher Windows versions and to use different API functions based on the minimum supported Windows version.
The OpenFileName structure would be an example of defining additional members for _WIN32_WINNT greater than or equal to 0x0500.
It even seems that _WIN32_WINNT is used for the majority of version checks in header files.