Once the configuration of the subsurface is known, it is possible to interpret the long history of events that formed the rock bodies in the arrangement we see them today.
Indeed, geologic cross sections are windows into the history of the Earth! They provide scientific evidence that the high plateau into which the Grand Canyon is cut was once at a much lower elevation- and even covered by shallow oceans.
Knowing ages of rock bodies and geological structures is also useful in determining geologic hazards. For example, few of us would want to live on an active fault that produces earthquakes!