Planes usually have two important parts: A main ability which can be a continuous effect, such as Tazeem in Zendikar (whose ability is that creatures can't block while this plane is active), or a triggered ability, such as Undercity Reaches from Ravnica (which allows the controller to draw a card each time one of their creatures deals combat damage to a player)
Its a way to get free mana from a card in your hand that you can turn into more mana with cards like Manamorphose or Pyretic Ritual
This is designed to help you find Hydras which might be useful outside Kindred construction
Thankfully, discarding isnt a major drawback thanks to the strong recursion of Zombies and Graveyard-based abilities
is also discounted as part of the Duskmourn Commander Deck bundle at Amazon, which is currently down to $189.20 for four decks (dropping from $242.97)
It doesnt show up as much these days since creatures are a lot bigger and decks are a lot faster, not to mention that a number of other cards may have been banned
