WWSAPI is a native-code implementation of WCF, providing network communication functionality by supporting a set of the WS-* and .NET-* family of protocols. WWSAPI is designed to be used by components/applications which fall into one of the following categories:
- Native code mandate
- Require minimal dependencies
- Require minimal start-up time
- Memory constrained environments
It is mainly designed for performance and memory usage while provided WCF interop, so uses an order of magnitude less memory that a C# implementation of equivalent functionality while handling a request load two to three times higher. Reference
It currently ships with and hence requires Windows 7 or Windows Server 2008 R2 for use, limiting its usability. Microsoft can provide versions for older releases but this requires a contract to that effect with Microsoft for its redistribution.