As a character explains, the small team at Game Freak had difficulty cramming all the data for Pokémon Gold and Silver into the limited memory of Game Boy cartridges. Thankfully this “amazing guy” (Iwata, who was well-known for his programming skills) wrote software that helped the content fit. You likely know what happened after that — Gold and Silver went on to become monster hits, and Iwata rose quickly through the corporate ladder to become president in 2002.
There are other development mini-stories if you bring creatures from other past games into Ultra Sun and Ultra Moon, so it’s worth mixing up your roster if you want to see absolutely everything. The Iwata ode is clearly the highlight, though, and shows just how extensive his legacy is.