Why not add files without rebuilding the program

I have a problem understanding the utility of order-only dependencies=20
in make.

For example, consider

obj/main.o: main.c | obj/
    gcc -c main.c -o obj/main.o
=20
obj/:
    mkdir -p obj

obj/ is built before obj/main.o, but updates to obj/ (e.g., adding=20
files) won't trigger a rebuild of obj/main.o.

Why would one add files if the program will not be rebuilt?  I think
that if files are added to a build directory, they are included because
they are needed for updating the program.