The force is the same, but the pressure isn't. In the case of the needle, the entire force is concentrated on its point; in the case of the nail the same amount of force is distributed over the larger area of the blunt end. So, though we exert the same force, the needle gives a much greater pressure than the blunt nail.
When we speak of pressure, we must always take into consideration, beside forces, also the area upon which this forces act. For example, when we are told that a worker is paid 20 dollars, we don't know whether this is for a whole year or for one month or just for a week.
Skies easily take us across fresh snow; without them we fall through. Why? On skies the weight of your body is distributed over a much greater area. Supposing the surface of our skies is 20 times more than the surface of our soles, on skies we would exert on the snow a pressure which is only a twentieth of the pressure we exert when we don't have skies on.
For the same reason horses used in marshlands are shod in a special fashion giving them a wider supporting area and lessening the pressure exerted per square centimeter. For the same reason people take similar precautions when they want to cross a bog or thin ice, often crawling to distribute their weight over a greater area.
Finally, tanks and caterpillar tractors don't get stuck in loose ground, although they are very heavy, again because their weight is distributed over a rather great supporting area. An eight-ton tractor exerts a pressure of only 600 grams per square centimeter.
This is why a sharp knife cuts better than a blunt one; the force is concentrated on a smaller area of the knife edge. To sum up: sharp objects prick and cut well, because more pressure is concentrated on their points and edges.