Starter shingles are installed on the eaves of a roof. Their purpose is to waterproof the rakes & eaves, minimize shingle blow-off by wind, and provide a finished look.
A basic, 3-tab shingle can be used as a strip of starter shingle by cutting off the tabs. However, this can lead to improperly positioned adhesive that does not provide adequate wind resistance. A dedicated starter strip is often required for the roof manufacturer to issue a warranty on the new roof.
GAF Starter Strips: Pro-Start Data Sheet