Why do wounds form scabs?
The short answer
Platelets and proteins form a clot, and the dried surface of that clot becomes a scab that protects the wound.
How it works
After bleeding is limited, skin cells repair the area underneath. The scab usually falls away as the new surface forms; picking it can reopen the wound.
A familiar example
A scab is not the final skin; it is temporary protection while healing continues underneath.