It is essential to plug leaks because loss of pressure in the blood vessels would reduce the efficiency of the pumping system.
Component: Platelets (thrombocytes)
Function: Platelets circulate in the blood and help clot the blood at the site of injury, thereby plugging the leak and preventing further blood loss.
Source: Life Processes, Chapter 5, Section 5.4.1 – Maintenance by platelets
The question has two parts — (1) reason beyond blood loss, and (2) name + function of the component. Examiners expect the pressure point specifically (not just "to stop bleeding") and both the name platelets and how they work (clotting). Mentioning "circulate around the body" adds accuracy but keep it concise for 2 marks.