The county is normally the largest territorial and political subdivision of a state. Counties vary greatly in population and land area.
When many Midwestern and Southern states were mapping out counties in the nineteenth century, the idea was that residents living in the farthest corners of a county should be able to get to the county courthouse and back by horse and buggy in the same day.
That is why states in these regions have so many relatively small counties.
The county courthouse was the center of government, serving as a headquarters for law enforcement, record keeping, and road construction, as well as courts. The towns where the county courthouse is located are called county seats.
For further reading, see the following study links below:
Of interest, find out the Top 10 Reasons to Study Geography, shared by cgeducation.ca.