Rocket's Admin. is a card that, and this is no exaggeration, every RS-PK deck plays, regardless of what type of Pokémon or strategy they're using. Think Sol Ring in Commander, if you play Magic: the Gathering.
What can this amazing card do?
If your opponent draws a lot of cards, perhaps because you mulliganed a lot, or they played Steven's Advice, or they used Holon's Castform's Delta Draw, Rocket's Admin. neutralizes their card advantage.
If your opponent is trying to hold a card in their hand, waiting for the right moment to use it, Rocket's Admin. disrupts their plan. They may get lucky and draw the card they shuffled in again, but they're more likely not to. A prime time to use Rocket's Admin. with this goal in mind is when your opponent searches for a card, but does not play it immediately.
Early in the game, Rocket's Admin. can function as a draw Supporter. If you've played out the rest of your hand, or your hand is full of dead cards, just Admin and treat it like a Professor Oak's Research or Copycat. The draw Supporters in RS-PK are much weaker than those in later eras, so while N/Iono might be a second- or third-best draw Supporter in those formats, the functionally identical Admin is solidly the best in lower-powered RS-PK.
Later in the game, when your opponent has few Prizes left, Rocket's Admin. can cut down their hand and force them to topdeck either exactly what they need, or a draw Supporter, to get back in the game. (Pidgeot can Quick Search out of this, which is what makes it so strong!)
Skilled players will try to discard dead cards (to Holon Supporters, for example) or play Trainers for no effect (like Swoop! Teleporter or Great Ball when there are no more Basics in the deck), just so they don't draw them again after getting Admin-ed. Playing useless Pokémon to the Bench is a lot riskier, as it strengthens opposing Steven's Advice, or worse, Pow! Hand Extension...