As an example, I have a package that uses a C library which depends on the presence of libgmp and libmpfr, both of which are large and come with Julia. On my Linux, libgmp.so and libmpfr.so are found in /usr/local/lib/julia/. If libgmp.h and libmpfr.h were found in /usr/local/include/julia, then I would not need to install duplicates or go through machinations to determine which version of each is in use and find the appropriate headers.