Itching is caused by very light motion on your skin. It probably evolved so that you will notice very slight movements from insects on your skin so that you can swat or scratch the area to remove them.
After your wound seals with a scab, the wounded region is filled with a fiber mesh by specialized cells known as fibroblasts. These meshes will slowly pull together over time as the wound heals. However, this small pull sometimes triggers the same nerves that send out the itching signal! This makes you want to scratch at the scab which is not conductive to healing.