In Pokemon GO, Dark-types are known for their offensive capabilities. In the mainline games, the Dark-type was introduced to counter the then-dominant Psychic type and did its job very well.

Even swapping Grimmsnarl's types, so that Hatterene would be Fairy/Dark-type, would have been preferable. Hattrene's Gigantamax form is even referred to as the "Raging Goddess," capable of sensing ...

Overall, Fezandipiti ex is not just the best Darkness-type Pokémon in the format currently, it is also among the most powerful Basic Pokémon in the game. Pokémon TCG: 10 Best Dark-Type Pokémon ...

Plus, Feraligatr already learns plenty of Dark type moves such as Bite, Crunch, Fake Tears, Snarl, and Lash Out. A secondary Dark typing would also give its Dark attacking moves the STAB bonus.