The Iron Giant/WMG

The Iron Giant is a self-replicating machine similar to a Berserker
That is why he eats so much metal and why there were more than one of him blowing up the planet in the deleted dream scene. The Giant's creators sent them out to destroy every other planet (or just all life) in the universe and they were programmed to land on a planet, consume copious amounts of metal, replicate, destroy the planet, and move on with each one going to another planet and starting the process again. The Giant forgot all this when he got zapped by the power station and banged his head.
 * Another idea by the same troper, and one more worthy of the title wild mass guessing, the giant is a weapon of peace! It was designed to land on a planet, provoke them by eating all the metal, and then see how the natives react. If the natives react by attempting to destroy the giant then they are too hostile and must be destroyed. If they attempt to make contact with it and communicate with it then the giant doesn't destroy them. That way, every violent race gets weeded out!

The giant was another race's attempt to create a robotic Ultraman
His head clearly looks like a robotic Ultraman's and the weapons could easily be made for fighting kaiju
 * Not even close, bucko. Talk about your Poison Oak Epileptic Trees...

The song Iron Man is the sequel to this movie
At the end of the film, we see the giant restructuring himself after his encounter with the nuke. However, his mind was screwed up by the blast, and instead of wanting to be Superman, he is now a robotic super villain coming to "Kill the people he once saved." Of course, "Now the time is near/for Iron Man to spread fear."
 * What about the "traveled time" bit, then?
 * Technically anybody who lives through the passage of time is said to be "traveling time". Albeit the slow way.

The Iron Giant is designed to weed out peaceful planets from hostile planets
He's a probe sent by Well Intentioned Extremist aliens. The Giant only fights in self defense. So, the logic of the aliens is that if the inhabitants don't fire upon the Giant, then it's a decent planet to be contacted. However, if the inhabitants fire at it, they are destroyed with extreme prejudice. We survived its attack, but unfortunately, we still blew our ticket into The Federation. They might return in a couple hundred years to see if we've mellowed out.

The Iron Giant came from the same Galaxy as Ratchet and Clank.
His head is nearly Identical to Clank's, complete with the same facial expressions and shutter-like eyes. And he has a buttload of weapons that he pulls out of nowhere. He is obviously an advanced model of the type of robot Clank is with Ratchet's arsenal thrown in as a bonus.

The Giant came from Cybertron.
He came down in the form of a comet, and transforms near the end. It's just that his initial head trauma and amnesia lead to him neglecting to scan an Earth mode.

The Iron Giant is actually Russian
He was designed to be a weapon to kill the Americans with, that's why he has such an easy time learning human languages. It's just his programming got all fouled up by his unintentional crash and he no longer sees there to be much of a reason to fight the Americans.

The Iron Giant is actually a prototype Super Robot
To be more specific, it's a Super Prototype of Giant Robo intended for a possible space exploration or defence against aliens.

At the start of the film Mansley has been Reassigned to Antarctica
His colleagues at the NSA may have found him just as annoying and obtrusive as Hogarth does and sent him on a Snipe Hunt just to keep him out of the office.