What The Heck Was In The Letter In The Original Legend of Zelda?

Does anyone else wonder what the heck was in the letter the old man gives you to give to the old woman in the original 8 bit NES Legend of Zelda? Those unfamiliar with the game won’t know what I’m talking about, but those who played surely remember:

The old man gives you the letter so that the old woman (for some reason, all human inhabitants of Hyrule other than Link and Princess Zelda are an identical old woman and old man).

But…why wouldn’t she sell you the medicine without the letter? What could possibly have been in it? It isn’t like people weren’t supposed to know who Link was. He didn’t need an introduction. So, what was in the letter? A prescription? Blackmail material the old man was going to use on the old woman if she didn’t cooperate? It just doesn’t make any sense.

Sure, I understand that the point was to make you have to get something extra before being able to buy the highly valuable medicine, a video game goal. But, what was the point in the hypothetical world?

I can’t figure it out and it bugs me. It shouldn’t, but it does.