Regarding the Blink tutorial why is pin 13 being referred to as "LED_BUILTIN"? It's not a built-in LED at all; the tutorial asks me to supply the LED externally. It would be less confusing if it was just called pin 13 as in:

// the setup function runs once when you press reset or power the board
void setup() {
  // initialize digital pin LED_BUILTIN as an output.
  pinMode(13, OUTPUT);

LED_BUILTIN is the pin with the on-board LED (pin13 on most boards).

It's used in the blink example so that blink will work even on boards where the on-board LED is on a different pin.

For use in a tutorial, I agree with you, it's more clear to use the pin number.

For use as a test sketch (which blink is frequently used for), it's more useful to have LED_BUILTIN so you don't have to fiddle with the sketch just to get a test sketch that you can use to verify basic functionality.