|Blood cells - The middle is a thrombocyte|
Blood clotting is important because any breach in a blood vessel, even capillaries (as happens when you cut yourself) can result in loss of blood. Too much blood loss and the body can't function, leading to shock and death. Normal clotting response is about 8 minutes if you go by my medical textbooks. The actual process involves chemical signals and varying proteins and pathways all interacting with one another, but the main thing that happens is that platelets stick together to act like a wall between the blood flow and the outside world (technically, the world outside the blood vessel - internal bleeding a serious complication of trauma and any blood loss outside the circulatory system is a problem).
Some people are born with clotting, or bleeding, disorders - hemophilia. When they are injured their vessels bleed easily or are unable to clot, resulting in the need for transfusions and exposure to various risks due to this.
So the platelets are important, as is the plasma that carries them. The water and nutrients in the plasma are used in various signaling processes in the bodies, like hormones that control heart rate and fertility, and the vitamins used in electrical signaling in the muscles. As well as the wastes, like carbon dioxide (produced by cells) and urea (nitrogen waste from protein metabolism) - the build up of which can cause disease.