The scale of the map itself (the area you’re showing) and the scale of the data you use both affect what your map will show. A classic example of how your choice determines the question answered is whether to show presidential election results by state or by county. While the state-level data does show a distinct national pattern, the county-level map reveals much more nuanced local and regional patterns. Map A answers the question, What is the pattern of states (and electoral votes) won by each candidate? Map B, about voting by county, better answers the question, What is the distribution of Republican and Democratic voters in this election?
Of course the area you’re analyzing—your town, county, region, or state, for example—often determines the scale of data you use. But even at the city or county level, you might be choosing between mapping information using census tracts, block groups, blocks, or even lots.