For instance, I used this trick to create a shared library wrapper (something that calls dlopen, but provides stubs so the program thinks it's just calling a build-time-linked library).

Macros defined the individual functions themselves, in one place, (what are their names, arguments, and which library). A regular #include of these macros provided the declarations to the program. The shared lib wrapper module #defined the definition version of these macros prior to the #include, which expand to the invocation stubs.

It makes sense because the users who maintain it just have one place to add a new function; they don't have to do copy and paste programming to write the invocation wrapper.