The best apps give users elegant solutions to precise needs—the more focused, the better. As you plan your app, think in terms of actual use cases or scenarios—brief story lines that cast the user as hero completing specific tasks in specific contexts. Like any good yarn, your app's story should answer "the five W's" that budding news reporters learn to pack into their lead paragraphs: who, what, when, where, and why. "Who" identifies the audience for your app, "what" identifies the actions they'll take, "when" and "where" zero in on context, and "why" describes their motivation and goals. By focusing on these story elements of your use cases—especially the why—you'll uncover your app's tapworthy conditions, the moments your users will need your app's superpowers.
Here's the hitch: in order for this to work, your use cases have to be plausible. There has to be an audience of people who not only want to do what you describe (the "who" and the "what") but more important, they have to want to do it on their iPhone (the "where" and the "when"). It's all too easy to lose sight of this mobile context. The iPhone is, after all, a full-fledged computer with an Internet connection and a grown-up processor capable of all kinds of complex tasks. For folks used to designing websites or desktop software, it's tempting to think of the iPhone as a "regular," if downsized, computer. While that's true in a technical sense, it doesn't translate to real-life use.
People behave very differently when using mobile apps on the go than we do when typing away at our desks. Just because you can put sophisticated software or a complete content reference on an iPhone doesn't mean anyone will actually want to use it there. The iPhone's form factor and relatively underpowered processor mean that the device is better for some uses than others. That means the "why" of your use cases has to embrace not only why users would use your app's features or content but why they'd use it on an itty-bitty handheld device.