You arrive at a trader, and find that he wants something specific - that you don't have with you. You can't buy any or sell any.
So, I want to basically "Give myself a mini-quest to go make this thing". I don't *have* the bread. I'm looking for some mechanism to do a fast-and-easy "Take a note" so that after I've dodged polar bears and trekked through the catacombs back to home base, (and saved the game, spent a week with other things, and come back), I know what the heck I was doing.
Ok, I'm standing here in the woods with a stack of spelt bread. Why? ... Oh, trade probably. Which trader? ...
Yes, I have game-external tools to do that. Yes, I can do it in game with the regular writing. But a "Take a note" button, or toggle, something, would make the in-game immersivity and usefulness better, imo.