Final answer:
The primary purpose of platelets and fibrin is forming clots to stop bleeding.
Step-by-step explanation:
The primary purpose of platelets and fibrin in the human body is forming clots to stop bleeding. Platelets are small cell fragments that are attracted to the site of a wound and form a platelet plug, which is a temporary seal for a small opening in a blood vessel. Fibrin, a protein formed during the clotting process, creates a mesh that traps platelets and forms a clot.