You have 1000 SP and you transfer it to me, it's still powered up. In order to sell it I'd need to power it down.
There might be a couple of hired goons coming to break your legs and you need the money now.
I want to buy STEEM and power it up.
You advertise 1000 SP available for 750 STEEM. I accept.
I get a 25% discount on STEEM I already planned on powering up; you get instant liquidity.