I find it strange that this is such an issue for people. I only buy cargo I intend to sell. Why are you lugging cargo to stations that don’t buy what you have?
I get that you may want to switch ships and have a temporary storage, so you can put cargo racks in a tougher ship (for example), but storing cargo while you go off bounty hunting just seems unnecessary to me.
Several reasons. First is that sometimes you have cargo you don't want to sell - engineer unlocks, or tech broker unlocks. Once you have part of that, you're locked into carrying it around until you get all the bits to unlock something. If you fly a combat ship, you may only have 16t of space anyway (ie when you switch to a combat build, you're severaly limited by this).
Note this applies to corrosive cargo - I found a titan drive component, after that I was limited to running my build with the corrosive cargo bay.
Second, limpets count as cargo. Its PitA already buying more limpets, switching to a ship without a controller and then selling the limpets (assuming you can switch because "too much cargo") isn't liked by the game - if you do sell all limpets in a ship without a controller it soft-hangs the UI as I think it doesn't realise you can have limpets without a controller.
Thirdly, just convenience. Times I have flown a big cargo ship into port thinking I'll just do a bit of something else for a change of pace - maybe I saw a planet I want to explore before I forget it, so switch to the exploration ship and then I'd come back and continue my trade runs. But that's too much for the game to cope with. I'm forced to sell all the cargo or bypass playing the gameloop I fancied.
And, its just wierd the game forces this on you. The limpets things particularly bugs me.