Very few applications require completely custom code. Most applications can benefit from modularization and code reuse, and it's important to use modularization to your benefit so that you can build apps more efficiently.
There's a syndrome common to many developers (and even some companies): the not-invented-here syndrome. The afflicted developer (or company) is extremely hesitant to use code, algorithms, or any other assets that they themselves didn't create. It's easy to see where this syndrome comes from—it ensures that one can't be held liable for third-party code, one can't be sued for improperly licensing assets, and so on; but it also means that one must continually reinvent the wheel. This invariably results in buggier, less secure algorithms, as everyone is always attempting to build their apps completely from scratch.
If you've worked through other books on mobile application development, chances are you've already been introduced to modularizing your own code....