Mental Note
Self-mill that doesn't cost a card: the entire pitch. Stocking the graveyard usually means paying a tax somewhere, in life, in tempo, or in card advantage, and the cleanest cantrips have always sidestepped that by replacing themselves while doing something incidental. Here the incidental thing is the loaded graveyard. Two cards land in the bin and the spell draws a fresh one, so a deck that treats the yard as a resource (threshold counts, reanimation fodder, flashback fuel, delve discounts down the line) advances its plan for free while still seeing the next card. Threshold is the era's context: the design wanted blue to fill the graveyard quickly without bleeding cards, and a one-mana instant that mills two and cantrips does exactly that, on either player's turn, at the moment threshold is one card short. The card it draws can be the threshold-enabler you needed, the spell it pitches to the yard can be the one you want back later, and the whole transaction costs nothing but the mana. That is why this template keeps getting reprinted and re-skinned: a graveyard-stocking effect with no card-economy downside is one of the safest, most reusable shapes blue has, and the only real ceiling is how many cards you're willing to feed past your own library.
