A blood clot is important because it prevents more blood loss and possible bleeding to death. Blood serves crucial roles of transport(of red blood cells, biochemicals, food molecules), nourishment, immunology, etc. And a certain volume of it should be maintained in the body for proper functioning, circulation, and balance ( the adult body needs roughly 4-6L of blood). This is why bleeds could be lethal, as in haemophilliac conditions; a state where blood lacks a clotting factor (VIII, IX, or XI).
