Exactly.
One needs a range that has a low that gets you up the stiffest local hill without undue stress and a high to comfortably descend the same hill w/o spinning like a hamster in a cage. Cram as many gears in between as you can afford for efficiency and proceed. Requirements change with seat time.

Experienced riders have several easy to change cassettes w/ appropriate gearing for their conditioning and: Flat, Rolling or Mountain terrain.

-Bandera