Why do this, you might ask? Well the above board reason is that it allows Microsoft to tune and modify the system call layer at will, improving performance and adding features without being forced to provide backwards compatibility application binary interfaces (or "ABI's" for short). The more nefarious reasoning is that it allows Microsoft applications to cheat, and call directly into the undocumented Win32 subsystem system call interface to provide services that competing applications cannot. Several Microsoft applications were subsequently discovered to be doing just that of course.