I really liked the juxtaposition of the "Technology is Incredible" guy from Pallet Town in the original Red/Blue/Green versus Let's Go Eevee/Pikachu.
The side games are where all the cutting-edge development happens. Someone comes up with an idea, and builds a prototype. If it works well, it gets integrated into the main-line games.
Pokemon Snap tested, "can we render a pokemon in 3D". Pokemon Stadium I and II tested "can we do a 3D battle system". Eventually these made their way into the 3DS titles. More recently, the catching mechanics in Niantic's Pokemon Go made their way into the main-line Let's Go Eevee/Pikachu.