The UI of the application is one of its most important user facing components. It decides how the user is going to perceive the application. A smooth and fluid frontend goes a long way in defining the user experience with the application.
This requirement for a smooth user experience brings in the need for optimizing the application frontend, in that it provides an easy to use interface, fast response times, and fluidity in the operations. If we go ahead and look toward the web 2.0 companies such as Google, Facebook, LinkedIn, and so on, they spend a huge amount of resources just on optimizing their frontends to shave off a few milliseconds of the rendering time. That is how important an optimized frontend is.