Hidden Move/Commentary

The introduction of Hidden Abilities in Generation V greatly shook up Ability distribution, giving most Pokémon a hard-to-obtain Ability option that tended to be stronger than normal ones and stretched flavor more. Examples include Politoed's Hidden Ability being Drizzle, which was originally the signature Ability of Kyogre, and the distribution of Overgrow, Blaze, and Torrent to the elemental monkeys, as these Abilities are normally exclusive to starter Pokémon... who themselves have Hidden Abilities different to each family, allowing them to be more individual as well.

A Hidden Move is the same thing, but obviously, it's a move. That's selling the idea short, though, because move availability and, more importantly, move choice function a lot differently than Abilities. A Pokémon is allowed four moves out of a pool of dozens, if not over a hundred, with various means of learning moves and the ability to override them at any time. In contrast, Abilities are limited to one at a time, out of only up to two per Pokémon (plus the Hidden Ability), and they're permanent without using a specific item to switch them. This makes moves overall less defining to a Pokémon's identity than Abilities; however, in battle, access to a specific move or combination of moves can still make or break one's usefulness. A decent-powered attack early on can cement a Pokémon's place on the team, reliable status-inducing moves are excellent for catching wild Pokémon, and so on.

Introducing Hidden Moves creates multiple avenues to bolster a Pokémon's movepool:

* It grants a new option to Pokémon who cannot be bred moves. Certain Pokémon can't breed at all, and genderless Pokémon can only breed with Ditto, which cannot pass moves to offspring. Additionally, when breeding normally, the species of the child always matches that of the mother, meaning that male-only Pokémon cannot receive new moves from parents because they will never hatch from an egg with a mother who could pass those moves to it.
* It allows limited access to otherwise signature moves, which can create new niches for weak Pokémon. Using the example above, Politoed receiving Drizzle made it a staple of the competitive metagame for generations, because it alone enabled an entire archetype of powerful teams that relied on Drizzle's ability to set rain automatically. Many signature moves are also restricted to legendary Pokémon who aren't available in a normal playthrough and are banned from most competitive play, and this lets those moves, and the Pokémon who receive them, find a new lease on life.
* It stretches the limits of a move being considered flavorful for a Pokémon to receive it. For instance, Sceptile receives Wood Hammer as its Hidden Move. Wood Hammer is fairly specific in its flavor, where a Pokémon swings down a sturdy tree-like part of its body onto the foe. It makes some sense for Sceptile to receive it because its powerful tail looks like a tree, but the tail isn't qualitatively similar enough to a tree for it to get the move through straight level-up. Wood Hammer has never been available through any other means apart from breeding, and its base form Treecko is not remotely tree-like to justify receiving it that way. Only through a Hidden Move is Sceptile able to gain access to a powerful Grass-type attack that bolsters the effectiveness of its otherwise struggling physically-based sets.
* It grants access to event-exclusive moves that are otherwise nigh-impossible to have anymore. Events here refers to Pokémon that were given out through promotions, almost all of which were limited-time only, often region-exclusive, and ones with special moves unavailable through other means were mostly a thing in Generations II-IV, which is a long time ago. Competitive players don't usually have to worry about it because they can use a simulator that just allows the move or organize trades online for one from the event. More likely, they'll hack the game to generate a Pokémon with the move, which is tournament-legal because it's technically possible to do without hacking. A casual, non-hacking cartridge player is out of luck, though, and that happens to be most of the fanbase.

By putting emphasis on Hidden Moves and making them readily accessible (though still hard to obtain), Earth & Sky opens new possibilities for a Pokémon to become unique and useful without having to break flavor or give it an overpowered signature move and/or Ability.