First of all, sorry for how long this is. I can change it to an answer if it would make things easier.
There isn't actually an official reason to this that I can find, so I'm going to assume this is another opinion based question (and in my opinion, too many of these are allowed). That said, I'm leaving this as a comment instead of an answer (since this is just my personal take on what the answer could be). To me, the answer as to what these things are for and why they exist is simple...
Nintendo wanted there to be a surplus of small Electric-types with cute cheeks to mimic Pikachu so that they can market even more off of their mascot than they already do. They're using it as a way to appeal to people because they know how much their fanbase likes Pikachu. Ever notice that the little plushie section at the store almost always has these little "Pika-clones" on sale alongside the starters?
It's a marketing ploy, a gimmick made to sell propaganda the same way mythicals exist for the sole purpose of cashing in profit via movies. Nothing else. However, you do bring up good points. They managed to use most of these clones for a unique purpose, which would also be a clever marketing ploy to call attention to the new mechanics via Pokemon viewers are most likely to be fond of. I think that Pachirisu showed off gender differences, which were introduced in Gen 4. It might also be because Pachirisu is allowed in Amity Square. The "theme" for Emolga could possibly be that you can find it in rustling grass. Emolga is also the only Pokemon to have a base-stat total of 428. Togedemaru has a signiture move, Zing Zap. Togedemaru also has access to the most egg-moves of all Gen 7 pokemon.
