Perhaps I don't understand it, but I think parent poster gave bad example of what s/he tried to say.

My understanding is that if you had $100 and you purchased stock for $100 and it went up in few hours to $120 and you decided to sell it, so you can purchase something else. You can't you have to wait 2 days.

Of course if you have enough money you will have buffer to account for that, but it makes it harder to do day trading when everything is delayed by 2 days.