Header files can sometimes contain very large amounts of source code (for instance, the header files windows.h and Cocoa/Cocoa.h on Microsoft Windows and OS X, respectively). This is especially true with the advent of large "header" libraries that make extensive use of templates, like the Eigen math library and Boost C++ libraries. They are written almost entirely as header files that the user #includes, rather than being linked at runtime. Thus, each time the user compiles their program, the user is essentially recompiling numerous header libraries as well. (These would be precompiled into shared objects or dynamic link libraries in non "header" libraries.)
