The below description is taken from LEGO.com.
"Disassemble! Demolish! Deconstruct!"
The Demolition Dummy exists to take things apart. He is compelled to disassemble everything that he comes across into its component parts, leaving piles of pieces behind wherever he goes. Although he doesn't seem to be very bright or creative, he is extremely persistent, and it's difficult to stop him once he starts trying to deconstruct something. No matter what anyone else does, he always seems to find a way to accomplish his task.
Having the Demolition Dummy and the Robot in the same place together is like watching a perpetual motion machine - one constantly builds while the other un-builds, over and over again. No one really knows what motivates the Demolition Dummy to take everything he sees apart, but he certainly seems happy about it when he's done....until he spots the next thing to disassemble, and then off he goes!