Why is putting rock layers in order by age useful to geologist?

Rock layers in order is basically natures calendar.

The highest layer is the newest formed layer 
While the deepest layer was the oldest and probably has been there from some time
And everything in between goes accordingly

So putting rocks in order by age helps scientists better understand what was there before what and how the rocks changed over time.

Hope this helps!
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Connecticut, delaware, maryland, massachusetts, new jersey, new york, new hampshire, north carolina, pennsylvania, rhode island, south carolina, virginia