Some projects cannot afford to have their implementation visible to the client programmer. It may show strategic information in a library header file that the company doesn’t want available to competitors. You may be working on a system where security is an issue – an encryption algorithm, for example – and you don’t want to expose any clues in a header file that might help people to crack the code. Or you may be putting your library in a “hostile” environment, where the programmers will directly access the private components anyway, using pointers and casting. In all these situations, it’s valuable to have the actual structure compiled inside an implementation file rather than exposed in a header file.