Continuity mistake: When the asphalt quakes in front of the church in the overhead shot, just as the huge gush of water shoots up from the ground, in the foreground a person is leaning way out their window aghast at the goings on. In the next shot that person is gone and the window is shut. (00:22:45)
Continuity mistake: When the intersection moves, a van is parked five feet away. When the intersection is destroyed forming a hole, the van is parked closer to the hole, allowing it to be thrown in. (00:23:10)
Continuity mistake: After the tripod appears from the hole in the ground, it bellows and pulls out its rays. In the next shot in the window's reflection, the tripod is seen pulling it out again. (00:26:00)
Continuity mistake: When Ray looks at the tripod, he stands beside a car that's plain and labeled Maxium. But when Ray runs away, that car is now covered with droplets of water. (00:26:05)
Continuity mistake: When Ray steals the van and speeds away there is a registration sticker at the corner of the windshield, on the driver's side. The sticker is gone when Ray starts shouting, "You saw! We're under attack!" when they're on the freeway. (00:30:10 - 00:32:15)
Continuity mistake: When Ray steals the van the front windshield is dirty and has water spots all over. Then in the following shots, it alternates between being entirely clean, or clean only in the area that wipers swipe clean. (00:30:10)
Continuity mistake: In the van, when Ray says to Manny "Get in, or you're gonna die," the next shot shows Rachel's face with tears in her eyes saying "What do you mean?" with a very distressed look. In next shot from the left side of the van, you can see Rachel on the right side of the screen with a different expression on her face, looking more calm, yet, in the next shot the look on her face is distressed with the look of crying again. (00:31:00)
Continuity mistake: When Ray is taking the van he's having a heated exchange with Manny, and Robbie's window is closed. However, just as Ray anxiously speeds away Robbie's window is wide open, for the dramatic close-up on Robbie's side view mirror, showing Manny's vaporizing demise, and when Robbie ducks his window is closed again. (00:31:10)
Continuity mistake: As Ray speeds away when Manny's shop blows up, Robbie shouts, "Holy s**t!!" and ducks. The rearview mirror on the van's windshield, in this close-up, is entirely different to the one seen a few shots later, when they're on the freeway. The mirror changes yet again later. (00:31:30 - 00:32:15)
Continuity mistake: In the exterior shot of Mary Ann's house, there are three large windows of the same size over the front door, and large bay widows on both sides of the door. In the interior shot, when Ray remarks, "They're never here," the window over the door is visible and it is one large center window with very small windows on each side. In the dining room there are two windows with wall in between, certainly not a large bay window. (Additionally, in the living room there is a fireplace, where there were two windows in the exterior shot - but some could say that those windows were boarded up and a fireplace was built in front of it.) (00:34:30)
Continuity mistake: When Ray, Robbie and Rachel arrive at Mary Ann's house, as the kids shout for their mom and Tim, the door to the basement under the stairs, as well as the den further up (beside the kitchen), are all perfectly visible. Later, after the house is destroyed, when Ray walks through the basement doorway, the walls, moldings, and so on are entirely different, and now there is an extra wall (perpendicular to the camera) to the left (viewer's) where there was none before. (00:35:10 - 00:41:40)
Continuity mistake: When Ray carries the Pennzoil box of groceries into Mary Ann's house, the goods change position between shots. (00:35:20)
Continuity mistake: Ray places the box of groceries down on the island counter in Mary Ann's kitchen. In the following shots the box (and goods inside), ceramic pitcher with utensils, tray of napkins, peanut butter, bread and bag, repeatedly, and very noticeably, change positions. (00:35:50)
Continuity mistake: When Ray empties the bag of bread, nine of its slices fall out, and he drops the plastic bag (the rest don't fall out). In the next shot there are seven slices total (not nine), and Ray tosses one of them near the box, since he needs only six. But a few shots later two slices appear near the box, together with the six that Ray uses to prepare for sandwiches, totaling eight. (00:36:25)
Continuity mistake: When Ray throws the bread with peanut butter at the window it lands dead center. However, in the next close-up it's off to the side of the glass, in order for Ray's and Rachel's reflections to be seen clearly. The slice of bread differs as well. (00:37:15)
Continuity mistake: In Mary Ann's basement, when Ray and Robbie are looking out the small window, as the aliens begin their attack, Rachel runs over and Robbie hugs her tightly. However, in the next shot of Robbie and Ray, Rachel is gone, yet in the very next close-up Robbie is still hugging Rachel. (The shot of Ray and Robbie without Rachel is cut from a previous shot, before she runs to them.) (00:39:30)
Continuity mistake: As Mary Ann's house shakes during the alien attack, when Ray shouts, "Where do we go? Robbie, this is your house. Where do we go?" Robbie is hugging Rachel. In this shot Rachel's hair is extremely curly, whereas in previous and following shots it's barely even wavy. (Most likely she's the body double.) (00:40:15)
Continuity mistake: When Ray and the kids are hiding out in the mom's basement, they take cover in a small utility room that goes pitch black when Ray shuts the door - even though there are windows above the girl's head that emit light before he shuts the door and after he opens it to leave. (00:40:35)
Continuity mistake: When Ray heads upstairs from Mary Ann's wrecked basement, there is a mess beside the stairs. However, in the next shot looking down, there's no mess near the stairs, furniture differs, and now there's a wicker hamper that appears out of nowhere. (00:41:35)
Continuity mistake: The airplane wreckage and debris all around Mary Ann's house differs between shots. (00:42:00)
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.