Why are there different leagues for trainer battles in Pokemon Go? Because Niantic is familiar with the game’s competitive scene, that’s why. The split allows players to choose between assembling a team of Legendary and elite Pokemon to use in the uncapped arena or having a much broader field in the lower CP ranges, just like how competitive play in the classic games tends to divide up individual monsters into tiers according to use. If you’ve ever browsed Smogon, this should feel familiar.
The development dispatch accompanying the trainer battle release also goes into detail about the emphasis this places on in-person contact, since only the highest-ranked friends can face one another at a range. There’s also the inclusion of an additional charged attack for your monsters to add another layer of strategy to the game, which should bring in the classic feeling of fighting your friends only to have them deploy monsters to counter all of your favorites and then laugh at you. Hey, they are proud of recreating the feel of classic trainer battles, it’s only natural.