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.
The "you can't part" here is what's not clear. My understanding of non-margin trading is that you would be able to sell for $120. What you would not be able to do is then purchase something else with that $120 until T+2 when it settles and the money is in your account again. You technically don't have that $120 until the settlement.
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.