The package is meant to add support to edit and compile C source files almost as is done with Pascal sources, but IIRC it's almost "alpha" state: it has a C completion "plugin" for the editor and little more.
Yes, for Pascal programmers it's mostly useless but it can be handy for mixed language projects or if you're doing a C-to-Pascal port, for example.
OK. So it's very useful for me. But it's totally failed. Lazarus already has highlighting support for C source. The most important thing this package should do is enable compiling of the C source the same way as Pascal source or at least add an menu entry under Tool to allow compiling of the C source. I see none.
If you want C support, first you need highlighting support (done), later compiling of individual C source (I don't see anything), then having C project type template added to New Project (I also see none), after that is C project building (should be the same way as Pascal project), the last is C code completion. I think he's doing everything reversed or at least the package description is misleading.
C code parsing and code completion is hard. People now use something called libclang to do the heavy work for them. Professional IDEs also do that. If the author could add libclang integration package he's a real hero.
p/s: don't take thing serious, I didn't ask anyone to do anything he or see doesn't want to do, I only show how it should be done.