I don't see any benefit of storage and what wrong of turning in the cargo from a mission to do something else?
Well then, you're just not trying hard enough! There are sooo many compelling reasons. Here's two really important ones that keep being brought up...
1. You just have to switch from your cargo ship right now to go do some combat, but you can't bear to sell that cargo for a single credit less than the maximum possible profit. Even though it's just a game.
2. You just have to switch from your cargo ship right now to go do some combat, but you can't bear to drop that mission because the rep loss would be devastating. Even though you don't really give a toss about the BGS, and it's just a game.
As for me, I look forward to the prospect of weaponising storage for my advantage. Go ahead, limit it to 200 - that will never satisfy the storage-for-profit advocates, but for me, being able to store 200 stolen VO/LTDs in a target station, so I can drop a few dozen a day into their black market to supplement other activities during a concentrated attack? Coordinated with even a half-dozen others and timed properly? Perfect.