Question

Why the pressure is always applied on the head of thumb pin

Answer 1

Tip of the pin: So, when you apply a constant force of X newton but the area of contact is very tiny then pressure applied would be very high and this is why the pin pierces whatever it is going into.  Head of the pin: Now for the head of the pin, you don't want the pin to pierce your finger. From Newton's third law we know that every force we apply generates an equal and opposite force. So the force generated by pin on your finger is X newton. Therefore area of contact is increased, which in turn decreases the pressure applied on your fingers so that you don't get a cut.