On Windows, the system headers contain annotations in SAL, a source code annotation language which describes function parameters in a richer way than is possible with the basic C language. These annotations can, for example, specify whether it is possible to pass NULL for a pointer parameter and how to pass sizes for array parameters.
cffi could use this information in several ways such as passing the correct size for an array argument instead of relying on the client code to specify this correctly. Exceptions could be raised by cffi when client code uses incorrect parameters such as passing NULL to a non-opt (optional) parameter.
The language is described here: http://msdn.microsoft.com/en-us/library/ms235402(v=vs.80).aspx
Simply allowing and ignoring SAL would also offer a small benefit. Currently the user should remove the annotations when copying system function prototypes into cffi-based source, but every edit made increases the possibility of error.