Suppose you've written some key data structure in its own file. Let's say it's a binary tree. The binary tree's functions are all implemented in bintree.c, and

Suppose you've written some key data structure in its own file. Let's say it's a binary tree. The binary tree's functions are all implemented in bintree.c, and its functions, defines, and struct are all declared in bintree.h. Now, you want to write some application-specific functionality that works on your binary tree. You need to pass a pointer to the binary tree to your new functions. Of course, you implement these new functions in app.c, and the functions are declared in app.h so that other parts of your application can call them. You're going to need to include bintree.h in app.h to ensure that the compiler knows about the binary tree's structure. Otherwise, you'll have to make sure to always include bintree.h before you include app.h. Is that really something you want to be bothered with?