Mostly copypasted from past posts, but since this keeps getting asked every couple days anyway...
Ship interiors would allow, among other things:
- EVA from and into other ships (you cannot get inside other ships if they don´t have interiors). This alone provides tons of gameplay opportunities. Pirating, salvaging, rescuing, kidnapping, resupplying, investigating etc etc.
The simple fact you could enter any ship (not just your own) would unlock many gameplay possibilities. You could retrieve the black box to find out what happened to a derelict ship, deliver supplies to ships, recover items from ships, investigate why a stranded ship lost comms, breach a stolen / hijacked ship to recover it or sabotage it, salvage parts from abandoned or wrecked ships, etc etc etc etc.
- Other players seamlessly boarding your ship (or yourself boarding someone else's) to go do some missions / exploration / walkabout / whatever together.
- Salvaging, scaveging parts from inside shipwrecks on the ground (again, you cannot get inside ships without interiors), or sabotage / theft missions from landed ships.
And then there is the immersion value, that will vary for each people of course, but its value is often underrated. For instance, RPG games do not need to have things like day/night cycles, seasonal weathers (or even just weathers), NPC daily routines, etc, none of that provides gameplay on it's own, but all of these make the game world feel much richer and usually enhance your experience while roaming those worlds. Again mileage will vary per person.