Header files are how declarations and definitions have been shared between source files in C++. Prior to standard library modules, you'd include the part of the standard library you needed with a directive like `#include <vector>`. Header files are fragile and difficult to compose because their semantics may change depending on the order you include them, or whether certain macros are or aren't defined. They also slow compilation because they're reprocessed by every source file that includes them.

Like header files, modules allow you to share declarations and definitions across source files. But unlike header files, modules aren't fragile and are easier to compose because their semantics don't change due to macro definitions or the order in which you import them. The compiler can process modules significantly faster than it can process `#include` files, and uses less memory at compile time as well. Named modules don't expose macro definitions or private implementation details.