For five mana he's able to exile himself, pulling every single 'Party Guest' card from your deck and putting them all onto the battlefield
We're especially loving this homage to Toph on the Collector Booster box
Scarlet Witch DARK MAGIC AND BIG SPELLS Bend reality to your will with the Scarlet Witch and unleash magic on a massive scale
The one Karakas can be a third Plains if your local metagame has a lot of Wasteland and not a lot of Reanimator, Show and Tell, and/or Control, but I have found the exposure to Wasteland to be worth it to include Karakas in the maindeck
Verdurous Gearhulk, Rishkar, Peema Renegade, and Nissa, Voice of Zendikar can't put counters on creatures
Of course, the main appeal is reducing the X-cost that's often associated with hydras