While I don't care for any particular Pikaclone very much, they make enough sense to me as addition if you look at them as game elements AND marketing tools simultaneously rather than scrutinizing those lenses separately.
They're usually there to highlight something like a new gimmick or gameplay mechanic of a generation (Pichu for Breeding, Plusle/Minun for Doubles, Dedenne for Fairy Typing, etc.) in the form of a single Pokemon, where if you want to use the Mon (effectively or simply at all), you will probably need at least some understanding of said gimmick. I'd even say this could apply in an inverted manner with Mimikyu, where the Totem fight and its Disguise ability shows players what happens if they just recklessly throw out their Z-moves and don't have any backup plan if the Opponent survives it.
Some immediate questions could be "wouldn't they learn these by simply playing the game?" and "can't they use other Pokemon for that?", which they probably could, but not quite as effectively. Pikachu is the series mascot, so you can assume that players who are new to the games or haven't been with the series for a while are somewhat familiar with how it plays and what it does compared to a new Pokemon. At the same time, using Pikachu itself for the new mechanics is dicey territory because you don't want to overburden any Pokemon with all the gimmicks at once (lest you get stuff like Charizard exhaustion between the repeated prominent Anime roles, 2 Megas, and Gigantamax in just a few Gens' time), so the Pikaclones let them evoke familiarity with Pikachu's playstyle and image without reusing Pikachu itself and risking/accelerating that kind of market burnout.
At the same time, the Pikachu context might highlight some practicality to the mechanic compared to what an objective explanation or tutorial might.
Two Examples:
- Pichu: "Oh, so Baby Pokemon can have better stats or moves that I won't get in the wild, since I've never seen a Pikachu with Dizzy Punch or this Encore move" (I'm assuming this is for new-enough players now or just contemporary players who wouldn't know Egg Movesets, just "Eggs = Moves you can't get normally)
- Dedenne: "Having another type on my Electric Pokemon is nice for letting them resist more stuff in battle like all these Grunts' Dark types. Oh, Fairy is super-effective on this Pokemon that resisted my Electric moves AND seems to hit everything else fine, I should use this more"
These are some immediate examples of how using understanding of Pikachu can convey use cases for the clone's mechanic/gimmick, while the similarity to the mascot makes it easy to throw merchandise and a minor anime role for it onto the shelves to see if it strikes marketing gold. If not, well then they can use it for a few more cutesy scenes with or in-place of Ash's Pikachu for episodes that call for it to be more of a serious Battling mon.
