Counties evolved with acquisition, settlement and population growth. For instance, in 1770 Augusta County VA stretched westward to the Mississippi River. Over time, other counties and then states were chopped off, causing settlers to 'move' but only on paper.
County/parish/borough records are extremely important to anyone researching family history as public records of early settlers remained with their courthouse at the time.of their actions. For instance, someone whose ancestors homesteaded the Ohio River valley in 1770 might find their early deeds and wills in present-day Augusta Co VA records.
Currently your county is very important in your everyday life; most especially if you live in a rural area. Each county has its own courthouse and specific laws regarding local issues such as policing, traffic, sanitation, courts, licensing, etc. conforming to any over-riding state rules. Local politics - county and state - are often a stepping stone for more important political posts.
America's disgrace is its poor participation in local - city, county, state - elections as those are the people who make most of the rules governing our day to day life.