Of course the main reasons behind things being the way they are would be for gameplay, since that generally takes precedence in the design, though lots of effort is put into making sure everything fits thematically as best possible, too. Also in a lot of cases you can come up with at least semi-logical explanations for situations/behaviors that may not be apparent at first (especially once you've seen much of what's possible in the world, and explored all of it).
Haulers do have specific tasks, though in most cases I didn't make it so they can leave the map (and have new ones arrive) because that would have a number of negative effects on the gameplay.
As for better weapons, there is mention of what's going on in the lore implying that a lot of the prototype technology is very new. Although you will find special robots using a lot of them later on if you explore more, just the common bots are in most cases still using older tech because there hasn't been time to create designs from the latest ones.
So yeah you could say research is a main purpose of the new weapons, as well as for robots close to the surface (though not the actual surface--you can learn more about all of this in game!).
