The suffix "-shire" comes from the Old English scir, meaning "a division" or "district." In Anglo-Saxon England, shires were administrative units. The Normans continued the system, and over time "shire" became synonymous with "county."
Scotland uses different terminology (e.g., Sutherland is a county but lacks -shire). The pattern is primarily an English phenomenon.
In short, "-shire" marks a historic administrative division, and its presence indicates an area's ancient organizational roots.