The event is doling out special 2km eggs that give you access to some rare Pokémon like Chansey, Larvitar and Mareep. Tyranitar has the highest potential CP of all non-Legendary Pokémon in the game, and even outclasses all available Legendaries besides Mewtwo. The event also gives you a chance to hatch Chansey and evolve it into Blissey, a high-CP Pokémon that's still deceptively useless. But the Gym meta no longer prioritizes anything really, because any strong Pokémon you put in there will just decay. Its high CP means that the game often auto-recommends it, but it won't do you much good in Raids.

Source: Forbes September 23, 2017 14:26 UTC