Visible crew/equipment: In Mary Ann's kitchen, just as Ray comments, "Ok, well you hungry? I'll get you some food," the clear reflection of a male crew member moving is visible in the glass window behind Ray. Then, just as he says, "This is ketchup and mustard," another male crew member, who is crouching, is visible directly in front of Rachel's reflection. (00:35:45)
Visible crew/equipment: When Ray and Ogilvy play tug-of-war with the gun, as the aliens rummage around the cellar, Ogilvy finally pulls it away from Ray. In the next close-up of Ogilvy and his gun, the boom mic dips down for a moment, but it's gone in Ogilvy's next close-up. (01:28:15)
Visible crew/equipment: After the ferry disaster, and close call with the car, the trio are underwater swimming up towards the surface. When Robbie turns his head to the side the regulator in his mouth is visible. (01:03:45)
Visible crew/equipment: When Ray is fleeing Mary Ann's destroyed house in the minivan, right before he says, "Robbie, get in," you can see a cameraman and camera in the reflection of the van window, right before it stops.
Visible crew/equipment: When Robbie gets out of Tim's car there are two of the crew's chalk marks on the sidewalk - one long blue line in front of Ray and an orange arrow near the trunk of the car. They are gone when Rachel leaves the car. (00:05:00)
Visible crew/equipment: After the plane has crashed Ray walks out of the house and towards the aircraft engine, as he walks behind the engine and before he comes from behind the engine, look at the right and to the rear of the engine, you will see a crew member move out of shot.
Visible crew/equipment: During the ferry disaster, while Ray, Robbie and Rachel are in the water, when the car heads toward them a vertical cable is visible at the left side of the car, beside the side view mirror. (01:03:30)
Visible crew/equipment: When the first ship is beginning to emerge from the ground, we see a shot of two men on top of a swaying scissor lift, notice in the close-up there is nothing between them. When the shot cuts to a wide view, a camera covered by tarps appears between the two men.
Visible crew/equipment: When Tom Cruise comes back in off the street covered in ash, he slumps down by the refrigerator and very quickly the boom mic falls down into shot almost hitting Tom on the head.
Visible crew/equipment: When Ray, Rachel and Robbie run aboard the ferry, and the camera follows them from behind, its shadow can be seen on the right side of the screen projected on cars and people. (01:03:39)
Visible crew/equipment: As Ray drives and Rachel screams, the camera crew and equipment are reflected in the window.
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.