Audio problem: The audio of the "SpongeBob SquarePants" episode on the T.V. doesn't match with the episode. (00:11:20)
Audio problem: At the beginning when Robbie runs up to tell Ray what happened with the car, and they're standing in the middle of the street, when Robbie says, "It opened up this hole or something," his lips aren't in sync with the words. (00:18:15)
Audio problem: When Ray carries Rachel out of Mary Ann's destroyed house, Ray and Robbie walk past some wreckage and are near the van, yet in the next shot they just start walking past that wreckage again. Additionally, the audio is not in sync with Ray's lips in the first shot. (00:45:40)
Audio problem: After Ray and Robbie's argument, as the soldiers drive by, when Rachel shouts, "Who's gonna take care of me if you go?" Robbie looks at Rachel and says something to her, but nothing is heard as the shot cuts away to Robbie picking her up. (00:50:15)
Audio problem: As Tom Cruise and the children walk towards the Athens NY ferry an older gentleman says, "We got it the worst, that's what I heard". If you look closely his lips do not match what he's saying. (00:56:30)
Audio problem: After Ogilvy is dead, when the alien snakelike giant eye surprises Rachel she begins screaming. As Ray hacks away at it with an axe, she stands at the wall screaming, and when the last long scream is heard, her mouth is closed just before Rachel runs to the stairs. (01:34:30)
Audio problem: In Boston, just after the military destroyed the tripod, in the tunnel Ray holds Rachel and we hear him say, "I love you. It's ok. It'll be ok," but his lips really say, "I love you. It'll be ok. It'll be ok." (01:45:10)
Audio problem: When Tom Cruise gets back to his house after the first encounter with a machine, Robbie's says "What happened?" but nothing is heard, then he says it again and is heard the second time.
Audio problem: When the tripod walks over the hill at the ferry scene it walks over two trees. However, the first tree it stepped on doesn't make a cracking noise as it should. The second one does.
Audio problem: When Robbie helps the people onto the ferry, his speech is out-of-sync whilst he is talking. Most noticeable when he says "Hold on".
Chosen answer: In the original George Pal version they were Martians and the reasoning for what they were doing was never explained. In this version, it's never explained where they come from, but their mission is simple, to eradicate human life from Earth, and use our bodies to fertilise the planet, probably so that they can colonise the planet for themselves.
GalahadFairlight
If it was to eradicate us they could have done that millions of years back, why now, so that doesn't add up.
You want to grow the substance (people) that grows your food source before using it. If they waited too much longer, they'd have a harder time because we'd have the technology to fight them back.
The reason which was apparently provided by Wells was that Mars was dying by lack of natural resources and that Martians needed a new home and food source.
They were waiting until the population grew large enough to sustain terraforming efforts. As they used our bodily fluids seemingly as a primary material for their terraforming.
It's an assumption that they could have eradicated us millions of years ago (which by the way would be long before we even existed). Maybe they didn't have the ability to transport themselves, only the machines. Maybe the original aliens all died. Lots of other options why they couldn't have done it.
They probably needed to wait for us to produce enough humans to use as fertilizer. Doesn't make sense to try to use several million bodies as fertilizer back then vs now with billions of people.