Why do people take Aery on Janna?

Because shields

Yes, you would do more damage with comet but Aery also makes your shields stronger while allowing you to deal more damage without needing to use W

Honestly people use Aery only for poke so much I forgot about the shield thing.

Less CD and also on shields

I personally like Electrocute if I intend to harass but Aery is the smart decision for playing to win. Shield boost and harass.

Someone did the math on it, in laning phase aery does more damage, late game comet really shines.

Kitty ear shields!