On a day with no wind at all, find a road to ride on that's dead flat. If you're lucky, you may find a gear that's right in the 'sweet spot". One that doesn't feel like you're either having to pedal too fast or to push too hard. More likely you'll find yourself "fishing" between two gears. The more closely your gears are spaced, the more likely you'll be able to find one that feels just right.

Climbing steep hills is just the opposite. You want an easy hill climb gear for going up and probably some faster gears for when you're speeding down the other side. That's going to require a more variation in your cassette cog sizes. Fortunately, I like wider gaps between gears in this situation.

Bike riders are always compromising between the two extremes. Some riders aren't satisfied with the cassette combinations that are commercially available and concoct their own custom cassettes.