int x = 42;
   int y = x / 2;
   double z = sqrt(x*x + y*y);
   return 1 + z;

Yes, functional programming matters. It lets you add two things together in C without worrying about allocating a destination operand for the result, whose clobbering won't affect anything anywhere else.

This sort of thing in turn makes it a heck of a lot easier to write OS schedulers, drivers, memory managers, codecs, ray tracers, database engines, ...