The same applies to equities - once the trade is made, the wheels are in motion. But actually transferring the securities takes up to 3 days.
One might ask "why is this important if the transfer itself takes days?"
The main reason is that it gives you certainty and closure. You try to make a trade for + or - X shares of Y at $Z and you get an essentially instant response. You don't have to wait around all day just to know if you've got a deal; you don't have to poke around looking for a slightly better price; you just make the offer and see it accepted immediately, and can move on to other things. (This is the sense in which I meant the word "clear" a few posts up: the trade is cleared from your to-do list, even if the securities take some time to actually move.)
There is a cost for the service of being able to move on: the HFT will likely capture a tiny bit of profit which could have gone to you with more effort. You've traded money for time and effort.
Having the trade agreed to quickly is a big deal even if it could take days to have any tangible assets to show for it.
Even regular bank transfers, i.e. transferring money from one checking account to another checking account, usually take a day to clear over here. I am talking about regular bank accounts in Germany, by the way. The economy manages to run just fine with this system.
As long as something as basic and fundamental as simply moving money from point A to point B is that slow, I really don't buy the argument that the real economy needs transactions on financial markets to clear that quickly. What's the point of having a trade clear in milliseconds if it takes me a day to transfer the money to a different account?