Cargo bays on spaceships: what's the point?

Like, is it just fluff? I'm looking at it and I can't think of a situation in which I, as a GM, would turn to my players and go "well y'all should upgrade that cargo bay because that way y'all can carry bigger stuff and make more money", since there isn't a trading / making money system like Traveller. I've been thinking of adapting the Traveller one for this, but as it stands... why would anyone upgrade the Cargo Bay?

Like, I get the others, since they either work to expand the amount of people on the ship or to add functions, like the crane or the vehicle bay, but I don't get the cargo bay.