One of the major challenges in mobile design is that there is not only one device or one platform. The static designs you create may be implemented and ported to multiple platforms and hundreds of devices. The more you understand these platforms, the features they enable, and how they interact with a given device, the better equipped you are to create designs that look great, perform flawlessly, and can be ported easily. This knowledge has less obvious benefits as well:
A good understanding of device and platform constraints frees you and the development team to experiment, be more creative, and make more-informed decisions throughout the project.
If documentation is part of your role, platform and device knowledge enables you to generate more relevant and useful reference documents for the team. A common understanding of what is possible also enables you to convey your vision to the entire team more easily.
Designs that can be built and ported on time and on budget make everyone happy.
Understanding the device begins with an understanding of the many platforms you may be building upon.