It looks like they hardcoded gcc as the compiler in the example. What about trying to port a program to a platform without it (or that calls it differently), that needs extra compilation flags, etc.?

A lot of the seemingly esoteric stuff in Makefiles is there to make porting feasible (and the rest is autogenerated by autoconf, automake, etc.). Not just which files an operation depends on, but e.g. the options used for building shared libraries correctly on this platform, paths for libraries, etc. While the build system will remember what it found as dependencies, it still needs to know what to find the first time. (It will note header files as dependencies, though.)