User Interface design plays a very important role as the translator in the communication between the user and the application's back-end or core functionality. It's purpose is to make the user feel comfortable interacting with and understanding your product. Most of a UI designer's job is spent thinking – the rest is executing the vision using the tools which work best for you.
Scaling your UI is becoming increasingly important as display technologies improve month-to-month. Not only does it provide you with infinite scaleability, vector paths are much easier to change, saving you valuable time.