Recall that platelets are involved in blood clotting, which prevents blood loss and the entry of microorganism.
Platelets are produced in the bone marrow which are just fragments of cells.
When we have wounds on our body, the exposure of the platelet to the air is that the platelets releases chemicals. In the blood there is a protein called fibrinogen which is soluble and with the chemical from the platelets, this turns into fibre which is solid. What happens is there will be a matrix/network of fibre molecules on the wound and onto this, solidifies the red blood cells and forms a scab. Beneath which, the cells will will be repairing the wounds and stops blood loss. White blood cell will attack any pathogens entering to wound.