For ages, people thought that a scab was necessary to the healing process - that it protected the wound from infection, made healing faster, more efficient and so on. Science has now told us that many times a scab does little to help a wound heal efficiently.
Scabs are necrotic tissue.
What the dead tissue does is form a hard barrier between the wound and the new layer of tissue being generated to close the wound. It actually slows down the healing process by blocking the tissue's regeneration process. The new layer of tissue eventually forms over the scab and this is what causes scarring. This isn't always a problem in most children and young adults because their tissue's natural power to regenerate itself is strong and naturally overrides some of the scab's scarring ability. In older adults, though, the tissue's ability to regenerate is not as powerful and scarring is more likely.