Flexibility to mission requirements, each individual vehicle is useful independently, then they can combine in 5s into a larger vehicle as needed, then into the Voltron mode if needed as well. You don't need a giant robot to do certain things. They were part of an exploratory force and the land, sea, and air teams did planetary exploration in each of those environments.
Looking at it practically, though (just for the sake of discussion), isn't it better just to have a separate giant robot already, instead of having to piece it together out of a bunch of separate vehicles? What if one of the vehicles is wrecked? What if they're too far apart to combine in time? What if the connectors are damaged or clogged by dirt or something? Sure, I can see the toy value of a combining robot/mecha, but is there any real-world practicality to such an approach?
We see so many combining or separating vehicles/robots in fiction. It's not just combining mecha -- we have things like the
Prometheus in
Star Trek: Voyager, one starship that breaks into three smaller starships. But are there any large machines or vehicles in real life that are made up of smaller components that can operate separately? Is there actual precedent for anything like this?
The first thing that comes to mind is a Saturn rocket or a Space Shuttle, but in those cases, the detachable parts are just boosters and fuel tanks that are discarded after a few minutes. The Apollo capsule had the Eagle module carried onboard, and they could operate separately, but the Eagle didn't really perform much of a role in the combined craft (aside from providing backup life support in the
Apollo 13 accident, IIRC). It was more just being carried along for the ride until it was needed, like the support craft on a ship. Of course, tractor trucks can have different trailers put on, but the trailers don't operate as independent vehicles; they're just being towed. Same with trains -- you can combine the cars in a bunch of different ways, but as a rule, only the engine can operate as an independent vehicle. Ditto motorcycles and sidecars.
I think that, in general, something that's trying to be two things at once tends to be less than ideal at either one. The
Prometheus has always struck me as a silly idea; three smaller ships are each going to be less powerful than the whole ship, and a ship that's made up of three separate pieces is going to be structurally weaker than a single integrated ship and have a lot of wasteful redundancies. You could probably make the same argument for a combining or shapechanging mecha. As a rule, if you need vehicles that can do, say, three different functions, it's probably better to make three separate, specialized vehicles than one that transforms into three configurations. After all, if the latter were really a practical idea, we'd see it used more in real life.