When you are at work in your office looking up porn on the computer while stroking your cock under the desk.
You suddenly realise you are going to cum and don't want to cum over your hand or your trousers / shirt, so you quickly pull your handkerchief out of your jacket pocket or trouser pocket, and place it over your cock - like a tent. the handkerchief (hanky) will soak up your load, thus avoiding having embarrasing stained clothes.
"I was looking up fat cum bitches at work today, and had to cum a hanky - you wanna see it"?
"Eww no that's discusting"!