Chain is a test armor, not actually to be in the game (until 1.3, where it is only obtainable by trading, so it is possible in survival to get it). It was a dev test left in.
Sponge was a feature that was dropped. Think Sky Dimension or whathaveyou. It just was dropped.
Well on the pc version they implemented Chain armor but never created an actual way to get it in survival yet they kept it. I'm not 100% sure about sponge though and seriously if you just go on the mine craft wiki you'll find it in a second, that's what it's made for!
What you said. ^
But, do we know why?
Sponge was a feature that was dropped. Think Sky Dimension or whathaveyou. It just was dropped.
My question was "why" not "what is it"..