The Eater is not essential every game like most other Cave Goblins cards, The Eater is not another cog in the machine.
The Eater is the insurance, when every other plan fails, a 5/9 body can be a winning plan on its own. The Eater's high life and damage for its decent cost can be extremely hard to deal with and it doesn’t really need much support, except for a snack or two. It can be used to kill commons, safely ignoring its ability for the most part, or used to attack champions, and just feed a Clinger or Climber if it fails to kill its target. Its real weakness is that it can be toyed around with, exploiting its need to have destroyed a unit on its turn, though this can be hard to do while having to simultaneously deal with the rest of the Cave Goblins.
Against factions that can bring a lot of units to the battlefield, The Eater may be the only way to make use of all that magic and feed on all those units.