I always see this sentiment here on HN. Everyone says "Why use blockchain, when you could just do X". Alot of the time when people say this, i don't think they

I always see this sentiment here on HN. Everyone says "Why use blockchain, when you could just do X". Alot of the time when people say this, i don't think they actually understand blockchain. There's a million here but i'll only ask one...How could you actively trade a 'contract' if it's not on the blockchain?