To scare away Jehovah's Witnesses and possibly terrorise small children. The idea is to build something reminiscent of Star Wars or The Matrix Revolutions out of old hydraulic JCB parts.
Once I've gotten hold of an array of broken digger arms, there are two problems to overcome. One, is making the walking movement functional, the other is getting the multiple arms to work simultaneously. I assume there is a limit to the oil pressure available, and it may be that multiple pressure thingies need to be installed.
Walking would not look like in the movies though. There's no way the legs would move fast enough to maintain its balance with momentum. It would have to be a slow, tilting walk to keep on its feet. Perhaps not as impressive, but hopefully it should be stable. At the very least it would need to be computer controlled.
Once it's walking, there's all kinds of fun to have in welding on weapons and tools. Some examples could be: flamethrowers, chainsaws, drills, paintball guns, huge gripping claws and, best of all, a big JCB scoop. For digging, and the like.