It reduces the resources available to your opponent.
Many of the Jake cards rely on the number of cornfields that are face-up. Husker knight stats for instance buff on the number of cornfields he has. The Pig destroys these, and makes Husker knights weaker.
Card cost also depends on the number of lands that are face-up. If you bring Jake down to 1 cornfield face up, he cannot play any cost 2 cards. No corn fields means none of his corn cards can be played!
Frankly I think the Pig is too limiting though. It harms the Jake Deck but is more or less useless in others!
AH! I see. Thank you. The Finn deck has a nice thing where in one hand i got the (i think its called) the "cool dog". and it protects both cards adjacent to it, which if you position your cards right Jake couldnt attack. But man the Jake deck seems so strong. Anyway, thank you again.