The Fisher trick: Write down the move you want to make, and then look in his eyes. There was a Fisher-Tal game and in the post mortem Tal asked: "Why didn't you play this move?" and Fisher said: "You laughed when I wrote it down"
chess rules

I believe that the new rule that states that you may NOT right down your move first was due to the advent of electronic scorekeeping. When using one of these tools, it allows a visualization of the board 1-ply ahead.
(If Martin Stahl said this, I apologize.)
Even in the USCF rulebook, it asks TDs to be lenient in enforcement with people using paper because it is not really aimed at people wrting with pen and paper.

What exactly is "so annoying" about someone writing a move down then erasing it?

Reb wrote:
The strangest rule I ever had enforced against me was at a Lisbon Open some years back . I was keeping score in the old descriptive notation and the arbiter noticed . He informed me that it wasnt allowed and I had to keep score in algebraic instead ! I was flabbergasted but complied anyway ...
I had some sort of experience. I wrote my moves in Hebrew, the arbiter said I couldn't do that, but my opponent allowed me to continue the way I did

What exactly is "so annoying" about someone writing a move down then erasing it?
I think it is the same as touching a piece.
you may agree that it is annoying if your opponent was allowed to touch Nf3, and release it and play b3.
I think it is the same.
I haven't met anyone who doesnt enforce the touch piece rule strict.
The strangest rule I ever had enforced against me was at a Lisbon Open some years back . I was keeping score in the old descriptive notation and the arbiter noticed . He informed me that it wasnt allowed and I had to keep score in algebraic instead ! I was flabbergasted but complied anyway ...