The traditional countdown — Rock! Paper! Scissors! — has one structural flaw: the last word is a throw name. Enough players have documented a tendency to throw Scissors after hearing the word that it's worth addressing with a neutral final command. "Shoot" has no association with any of the three throws. It functions purely as a release signal, like a starting gun. Your brain doesn't have a throw to reflexively match it to.
That's the practical reason. The deeper reason is that fair simultaneous reveals require a clean signal that both players interpret identically. If one player throws on the final syllable of "Scissors" and the other throws on a half-beat after, you have a sequencing problem that a referee or camera can resolve in a controlled setting but that creates real ambiguity in casual play. A sharp one-syllable command with no throw-word ambiguity reduces that problem.
The cadence used in WRPSA-sanctioned competition is a three-count pump followed by "Shoot" on the fourth beat. Both hands pump in sync for three beats and reveal on four. The pump serves two purposes: it's a physical commitment signal (you're demonstrating readiness by moving), and it establishes a shared rhythm that makes the reveal more reliably simultaneous than a verbal countdown alone.
Early throws — revealing before "Shoot" — are the most common timing violation in tournament play. They're usually not intentional. They're a product of anxiety and anticipation compressing the player's timing. Referees watch for them specifically. The pump-cadence makes early throws more obvious because the hand is visibly ahead of the rhythm.
The "Shoot" standard is now widespread enough that most players learning from a competitive context adopt it by default. It doesn't meaningfully change the game. It just removes one source of ambiguity and one potential (if small) source of advantage. At the level where tournament outcomes matter, removing small sources of advantage is exactly what rules are for.