Having more variety in the gameplay.
That's just it. A new capital ship in the game means a new 'boss' to be faced in early game, a new tough enemy that shows up in varying compositions in later-game fights, a new set of design challenges and playable builds, a new potential flagship to become skilled with, and so on. Hours of gameplay. Qualitatively different from using other ships, or facing them.
A new logistics ship may or may not get added to a player's fleet, and some numbers determining trip length will be slightly different. By mid-late game, fuel and cargo space are no longer major constraints anyways, though. You see it maybe once or twice in actual gameplay, and it works exactly the same as any other ship of the same class.
They take roughly the same amount of time to sprite out.
Having a more of a "signature" faction outlook besides the already existing Buffalo recolors.
I think it's more thematic for one ship to fill the role. "I want to carry as much supplies/fuel as I physically can, stealth and speed be damned" is a simple problem that can be solved with one fairly simple design each. Anyone considering designing a competitor faces competition from a tried-and-true design that already fills the role adequately, and already has a maintenance and manufacturing pipeline in place.
I've seen space sims go down both routes, with some giving every faction its own functionally identical superfreighter, and others plotting out sets of old designs that have survived the test of time and become ubiquitous across borders. I think the latter is more realistic and immersive - having the red faction spend billions of space-credits to make their own red megafreighter with 4100 cargo and 7.1 speed when they could just copy over the schematics for the yellow faction's megafreighter with 3900 cargo and 7.2 speed doesn't really make sense, and different nations in real life all tend to have broadly similar designs for big pieces of equipment whose jobs have been around for long enough.
