Traditionally on most Unix systems cc and gcc where two different compilers. Many times people would write code in ways that compiled with gcc, but not cc, so specifying exactly which compiler you want makes sense.
Users can use a different compiler by specifying it on the command line via e.g.
make CC=clang
CC=gcc
CC?=gcc
This is only relevant if you want portability to other versions of Make. I simply don't care about anybody who's not using GNU Make these days. Try it yourself: command line options override variables in makefiles unless "override" is specified for that assignment.