Continuity mistake: On the ferry, Rachel wears the purple/pink camouflage hoodie over her orange knit sweater with the colorful stripe sleeves, as usual. Now this is where things become strange. In the first shot as the trio come ashore, she is only wearing her orange sweater, she is not wearing the purple/pink hoodie - note the clear lack of fur hood. In the very next shot as they climb a bit more and then rest, Rachel is wearing the purple/pink hoodie - note the hood, but now it's on under her orange sweater. In the underwater shots she is also wearing it under the orange sweater. (This odd mistake has absolutely nothing to do with the very long wool cloak that Rachel wears in the next scene). (01:02:20 - 01:04:40)
Continuity mistake: Whilst Robbie is helping the people onto the ferry in the overhead shot, Ray holds Rachel beside a red car down below as they are both avidly watching Robbie's heroism. However, in the next close-up Ray is just now approaching the car, amongst a greater crowd of people. (01:02:20)
Continuity mistake: When we see the Hudson-Athens ferry boat captain from outside of his bridge, the video screens in each window above his head are positioned well below the tops of each window. In the next shot they're higher and we barely see them. (01:02:35)
Continuity mistake: In the interior shot, the Hudson Ferry captain walks toward and then stands at the doorway (seen clearly when he cast off earlier), as he stares at the growing whirlpool. However, in the next exterior shot he is standing at an open porthole window, staring at the water. (01:02:45)
Continuity mistake: After the ferry disaster, the trio swim to shore and climb up the bank. In the side shot Ray holds Rachel's left arm and Robbie holds her right arm. However, in the next back shot Ray holds Rachel's right arm, and Robbie holds her left. (01:05:00)
Continuity mistake: While the aliens rummage through Ogilvy's cellar, Ray and Ogilvy have a tense, silent tug-of-war with Ogilvy's gun, and their arm and hand positions differ between consecutive shots. (01:27:15)
Continuity mistake: In Ogilvy's cellar, in the close-up of Ray's hand, he holds it up to the red mist blowing through the two broken windows. In the next shot however, as Ray looks at his hand and realizes it's blood, the right window is suddenly blocked with a broken shutter and two pieces of wood. (01:29:15)
Continuity mistake: When Ogilvy shouts, "Ray! Ray! Ray..." in his cellar, Ray runs to him and there's blood splattered all over Ogilvy's face, but in the next shot the amount and pattern of the blood differs. (01:29:30)
Continuity mistake: When Ogilvy shouts Ray's name half a dozen times, Ray runs to him to quiet him down. The broken cellar window behind Ogilvy has shards of glass and red roots growing on it. Then in the next exterior shot, the shards of bloody glass are different size and shape, and the roots differ as well. (01:29:35)
Continuity mistake: When Ray tries to quiet Ogilvy, and then decides to kill him, the pattern and size of the sweat stain on Ray's shirt differs repeatedly in the following shots. (01:30:45)
Continuity mistake: When Ray places the blindfold on Rachel's eyes, while in Ogilvy's cellar, its position and folds differ in consecutive shots. (01:31:40)
Continuity mistake: In the scene right after Ray kills Ogilvy, Rachel finishes singing her song and as she's taking off her blindfold you can see that her hair is all messy and dry. In the very next shot her hair is all wet and neat. (01:33:00)
Continuity mistake: Ray leaves Ogilvy's house to search for Rachel and soon runs into the pickup truck, just before the tripod's tentacle lifts it. The massive red roots growing all over the pickup are very different in the next shot, as it's lifted in the air. (01:36:20)
Continuity mistake: When the tripod tosses the pickup truck, with Ray in it, it lands upside down and the windshield shatters. The size, shape, and characteristics of the hole and breakage greatly differ between shots. (01:36:25)
Continuity mistake: Whilst Ray is trapped in the upside down pickup, he looks through the windshield hole directly at Rachel, who stands on higher ground. In the next shot from behind Rachel, the landscape, wood fence, etc., to her sides are entirely different from the previous shot. (01:36:35)
Continuity mistake: As the tripod is attempting to have Ray sucked up, the soldier begins pulling Ray's arm down, trying to free him while he himself is being raised in the air. In the following shots, Ray's rescue is shown from three different angles, and each angle is distinctly different from the others. (01:40:00)
Continuity mistake: While up in the tripod's "holding cell" for humans, Ray spits the circular grenade pins into his hand, and their positions on his palm differ between the two consecutive close-ups. (01:40:15)
Continuity mistake: In Boston, when Ray breaks off a piece of the dried up root from the statue, the piece he holds in the next shot is completely different. (01:41:35)
Continuity mistake: In Boston, Ray and Rachel discover dead red weed on a statue. If you look behind them, you can see a space between two buildings that is clear, only sky. In the next shot, when the crashed tripod is being shown, the spot where there was just sky now has the tripod legs. Compositors forgot to add the tripod in the previous shot. (01:41:55)
Continuity mistake: When Ray finally arrives at his ex in-laws' house, in the shot from behind Mary Ann, Ray passes the two cars on his right and is near the corner, but a few shots later, when Ray puts Rachel down, he is standing beside the first car, back up the street. (01:46:55)
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.