Several people, most prominently and recently Andy Ihnatko, feel Apple over-emphasizes designing the iPhone for one-handed use. The general argument is there are only a handful of scenarios where one-handed use is necessary, whereas much of the rest of the time a larger screen would be more valuable.
This is a very seductive argument, one which I felt was intuitively wrong, but which I couldn't logically argue against. Most of the time one-handed usage is convenient, but not necessary. So why does Apple emphasize this feature so much?
Then my world changed a month ago with the birth of my first child and I'm reminded once again that context matters. Now I know.
When you have an infant child, having a light, thin phone you can hold and operate in your other hand is a godsend. The scenarios are endless.
Baby only wants to sleep in your arms? Are you holding baby with one arm and she makes a cute face and you want to snap a quick picture? Do you have to carry an infant car seat around while trying to contact your spouse to figure out where you're meeting? Carrying baby supplies in one arm while trying to respond to a work email? Are you trying to FaceTime grandparents by yourself?
Play this forward a few years and the scenarios will change, but the need won't. The toddler will simply be holding your hand while you walk, or you'll be holding him in your arm, just a bit differently than as an infant.
Without a young child, I had difficulty coming up with many cases where one-handed operation of a phone was truly necessary. Now I run into them every day.
I can't believe I'm the first to make this point, but I never hear it. Let me know what I've missed.