>>58176183They served two purposes.
First is nostalgiapandering to guarantee sales
Second was for the Let's Go games. They're obtainable through in-game trades there, and it's very obvious their new typings are there to fill gaps in the original Pokedex by giving you more Ghost, Dark, Ice, and Dragon types.
Either way Exeggutor, Marowak, Ninetails, and Raichu were all great