If you had yourself a little inkling that Yertle's throne is so much more than a throne, you're right on. First, his throne is really a stack of turtles. So yeah. This might seem like a no-brainer to you, but it's a Very Important Point. It symbolizes all of that hard, turtle labor that's gotten Yertle to where he wants to be—labor he'll never appreciate and never really see.
But, more than just that, the throne-stack also represents Yertle's relentless ambition, and all that he'll never have. Because that stack will never be high enough, and his appetite will never be satiated. It'll always be saying, feed me, Yertle. Feed me.