Why is it useful for historians to base certain studies on the analysis of historical periods?

What made this representation of the virgin mary revolutionary

Which statement in the passage best reflects sumer's opinion of the dred scott decision? a."i speak what cannot be denied." b."[the] judgment was sustained by a falsification of history." b."the opinion was more thoroughly abominable than anything history." i speak what cannot be denied when i declare that the opinion of the chief justice in the case of dred scott was more thoroughly abominable than anything of the kind in the history of courts. judicial baseness reached its lowest point on that occasion. you have not forgotten that terrible decision where a most unrighteous judgment was sustained by a falsification of history. -charles sumner, 1865

How did the united states ironically create a slaverybased market economy and democratic political system?